diff --git a/modules/openapi-generator/src/main/resources/php/HeaderSelector.mustache b/modules/openapi-generator/src/main/resources/php/HeaderSelector.mustache
index 5d62a7b5b9d..75e86d3a00a 100644
--- a/modules/openapi-generator/src/main/resources/php/HeaderSelector.mustache
+++ b/modules/openapi-generator/src/main/resources/php/HeaderSelector.mustache
@@ -1,6 +1,6 @@
 <?php
 /**
- * ApiException
+ * HeaderSelector
  * PHP version 7.4
  *
  * @category Class
@@ -18,10 +18,8 @@
 
 namespace {{invokerPackage}};
 
-use \Exception;
-
 /**
- * ApiException Class Doc Comment
+ * HeaderSelector Class Doc Comment
  *
  * @category Class
  * @package  {{invokerPackage}}
@@ -32,10 +30,11 @@ class HeaderSelector
 {
     /**
      * @param string[] $accept
-     * @param string[] $contentTypes
-     * @return array
+     * @param string   $contentType
+     * @param bool     $isMultipart
+     * @return string[]
      */
-    public function selectHeaders($accept, $contentTypes)
+    public function selectHeaders(array $accept, string $contentType, bool $isMultipart): array
     {
         $headers = [];
 
@@ -43,20 +42,13 @@ class HeaderSelector
         if ($accept !== null) {
             $headers['Accept'] = $accept;
         }
+        if(!$isMultipart) {
+            if($contentType === '') {
+                $contentType = 'application/json';
+            }
+            $headers['Content-Type'] = $contentType;
+        }
 
-        $headers['Content-Type'] = $this->selectContentTypeHeader($contentTypes);
-        return $headers;
-    }
-
-    /**
-     * @param string[] $accept
-     * @return array
-     */
-    public function selectHeadersForMultipart($accept)
-    {
-        $headers = $this->selectHeaders($accept, []);
-
-        unset($headers['Content-Type']);
         return $headers;
     }
 
@@ -77,22 +69,4 @@ class HeaderSelector
             return implode(',', $accept);
         }
     }
-
-    /**
-     * Return the content type based on an array of content-type provided
-     *
-     * @param string[] $contentType Array fo content-type
-     *
-     * @return string Content-Type (e.g. application/json)
-     */
-    private function selectContentTypeHeader($contentType)
-    {
-        if (count($contentType) === 0 || (count($contentType) === 1 && $contentType[0] === '')) {
-            return 'application/json';
-        } elseif (preg_grep("/application\/json/i", $contentType)) {
-            return 'application/json';
-        } else {
-            return implode(',', $contentType);
-        }
-    }
 }
diff --git a/modules/openapi-generator/src/main/resources/php/api.mustache b/modules/openapi-generator/src/main/resources/php/api.mustache
index 35955992f4f..a9893bf9e3f 100644
--- a/modules/openapi-generator/src/main/resources/php/api.mustache
+++ b/modules/openapi-generator/src/main/resources/php/api.mustache
@@ -60,7 +60,16 @@ use {{invokerPackage}}\ObjectSerializer;
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [{{#operation}}
+        '{{{operationId}}}' => [{{#consumes}}
+            '{{{mediaType}}}',{{/consumes}}
+        {{^consumes}}
+            'application/json',
+{{/consumes}}        ],{{/operation}}
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -151,6 +160,7 @@ use {{invokerPackage}}\ObjectSerializer;
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
 {{/-first}}
 {{/servers}}
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['{{{operationId}}}'] to see the possible values for this operation
      *
      * @throws \{{invokerPackage}}\ApiException on non-2xx response
      * @throws \InvalidArgumentException
@@ -159,9 +169,9 @@ use {{invokerPackage}}\ObjectSerializer;
      * @deprecated
     {{/isDeprecated}}
      */
-    public function {{operationId}}({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}?int $hostIndex = null, array $variables = []{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
+    public function {{operationId}}({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}, {{/allParams}}{{#servers}}{{#-first}}?int $hostIndex = null, array $variables = [], {{/-first}}{{/servers}}string $contentType = self::contentTypes['{{{operationId}}}'][0]{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
     {
-        {{#returnType}}list($response) = {{/returnType}}$this->{{operationId}}WithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}$hostIndex, $variables{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}});{{#returnType}}
+        {{#returnType}}list($response) = {{/returnType}}$this->{{operationId}}WithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}, {{/allParams}}{{#servers}}{{#-first}}$hostIndex, $variables, {{/-first}}{{/servers}}$contentType{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}});{{#returnType}}
         return $response;{{/returnType}}
     }
 
@@ -209,6 +219,7 @@ use {{invokerPackage}}\ObjectSerializer;
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
 {{/-first}}
 {{/servers}}
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['{{{operationId}}}'] to see the possible values for this operation
      *
      * @throws \{{invokerPackage}}\ApiException on non-2xx response
      * @throws \InvalidArgumentException
@@ -217,9 +228,9 @@ use {{invokerPackage}}\ObjectSerializer;
      * @deprecated
     {{/isDeprecated}}
      */
-    public function {{operationId}}WithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}?int $hostIndex = null, array $variables = []{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
+    public function {{operationId}}WithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}, {{/allParams}}{{#servers}}{{#-first}}?int $hostIndex = null, array $variables = [], {{/-first}}{{/servers}}string $contentType = self::contentTypes['{{{operationId}}}'][0]{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
     {
-        $request = $this->{{operationId}}Request({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}$hostIndex, $variables{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}});
+        $request = $this->{{operationId}}Request({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}, {{/allParams}}{{#servers}}{{#-first}}$hostIndex, $variables, {{/-first}}{{/servers}}$contentType{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}});
 
         try {
             $options = $this->createHttpClientOption();
@@ -367,6 +378,7 @@ use {{invokerPackage}}\ObjectSerializer;
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
 {{/-first}}
 {{/servers}}
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['{{{operationId}}}'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
@@ -374,9 +386,9 @@ use {{invokerPackage}}\ObjectSerializer;
      * @deprecated
     {{/isDeprecated}}
      */
-    public function {{operationId}}Async({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}?int $hostIndex = null, array $variables = []{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
+    public function {{operationId}}Async({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}, {{/allParams}}{{#servers}}{{#-first}}?int $hostIndex = null, array $variables = [], {{/-first}}{{/servers}}string $contentType = self::contentTypes['{{{operationId}}}'][0]{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
     {
-        return $this->{{operationId}}AsyncWithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}$hostIndex, $variables{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
+        return $this->{{operationId}}AsyncWithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}, {{/allParams}}{{#servers}}{{#-first}}$hostIndex, $variables, {{/-first}}{{/servers}}$contentType{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
             ->then(
                 function ($response) {
                     return $response[0];
@@ -428,6 +440,7 @@ use {{invokerPackage}}\ObjectSerializer;
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
 {{/-first}}
 {{/servers}}
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['{{{operationId}}}'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
@@ -435,10 +448,10 @@ use {{invokerPackage}}\ObjectSerializer;
      * @deprecated
     {{/isDeprecated}}
      */
-    public function {{operationId}}AsyncWithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}?int $hostIndex = null, array $variables = []{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
+    public function {{operationId}}AsyncWithHttpInfo({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}, {{/allParams}}{{#servers}}{{#-first}}?int $hostIndex = null, array $variables = [], {{/-first}}{{/servers}}string $contentType = self::contentTypes['{{{operationId}}}'][0]{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
     {
         $returnType = '{{{returnType}}}';
-        $request = $this->{{operationId}}Request({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}$hostIndex, $variables{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}});
+        $request = $this->{{operationId}}Request({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}, {{/allParams}}{{#servers}}{{#-first}}$hostIndex, $variables, {{/-first}}{{/servers}}$contentType{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}});
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -517,6 +530,7 @@ use {{invokerPackage}}\ObjectSerializer;
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
 {{/-first}}
 {{/servers}}
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['{{{operationId}}}'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
@@ -524,7 +538,7 @@ use {{invokerPackage}}\ObjectSerializer;
      * @deprecated
     {{/isDeprecated}}
      */
-    public function {{operationId}}Request({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#servers}}{{#-first}}{{#allParams}}{{#-first}}, {{/-first}}{{/allParams}}?int $hostIndex = null, array $variables = []{{/-first}}{{/servers}}{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
+    public function {{operationId}}Request({{^vendorExtensions.x-group-parameters}}{{#allParams}}${{paramName}}{{^required}} = {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}{{/required}}, {{/allParams}}{{#servers}}{{#-first}}?int $hostIndex = null, array $variables = [], {{/-first}}{{/servers}}string $contentType = self::contentTypes['{{{operationId}}}'][0]{{/vendorExtensions.x-group-parameters}}{{#vendorExtensions.x-group-parameters}}$associative_array{{/vendorExtensions.x-group-parameters}})
     {
         {{#vendorExtensions.x-group-parameters}}
         // unbox the parameters from the associative array
@@ -533,7 +547,9 @@ use {{invokerPackage}}\ObjectSerializer;
 {{/allParams}}{{#servers.0}}
         $hostIndex = $associative_array['hostIndex'];
         $variables = array_key_exists('variables', $associative_array) ? $associative_array['variables'] : [];
-{{/servers.0}}{{/vendorExtensions.x-group-parameters}}{{#allParams}}
+{{/servers.0}}
+        $contentType = $associative_array['contentType'] ?? self::contentTypes['{{{operationId}}}'][0];
+        {{/vendorExtensions.x-group-parameters}}{{#allParams}}
         {{#required}}
         // verify the required parameter '{{paramName}}' is set
         if (${{paramName}} === null || (is_array(${{paramName}}) && count(${{paramName}}) === 0)) {
@@ -578,9 +594,7 @@ use {{invokerPackage}}\ObjectSerializer;
             throw new \InvalidArgumentException('invalid value for "${{paramName}}" when calling {{classname}}.{{operationId}}, number of items must be greater than or equal to {{minItems}}.');
         }
         {{/minItems}}
-
-        {{/hasValidation}}
-        {{/allParams}}
+        {{/hasValidation}}{{/allParams}}
 
         $resourcePath = '{{{path}}}';
         $formParams = [];
@@ -649,21 +663,17 @@ use {{invokerPackage}}\ObjectSerializer;
         }
         {{/formParams}}
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                [{{#produces}}'{{{mediaType}}}'{{^-last}}, {{/-last}}{{/produces}}]
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [{{#produces}}'{{{mediaType}}}'{{^-last}}, {{/-last}}{{/produces}}],
-                [{{#consumes}}'{{{mediaType}}}'{{^-last}}, {{/-last}}{{/consumes}}]
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [{{#produces}}'{{{mediaType}}}', {{/produces}}],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         {{#bodyParams}}
         if (isset(${{paramName}})) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization(${{paramName}}));
             } else {
                 $httpBody = ${{paramName}};
@@ -687,9 +697,9 @@ use {{invokerPackage}}\ObjectSerializer;
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
index 914233b96fa..3fce701c54e 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
@@ -69,7 +69,14 @@ class AnotherFakeApi
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [
+        'call123TestSpecialTags' => [
+            'application/json',
+        ],
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -121,14 +128,15 @@ class AnotherFakeApi
      * To test special tags
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['call123TestSpecialTags'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Client
      */
-    public function call123TestSpecialTags($client)
+    public function call123TestSpecialTags($client, string $contentType = self::contentTypes['call123TestSpecialTags'][0])
     {
-        list($response) = $this->call123TestSpecialTagsWithHttpInfo($client);
+        list($response) = $this->call123TestSpecialTagsWithHttpInfo($client, $contentType);
         return $response;
     }
 
@@ -138,14 +146,15 @@ class AnotherFakeApi
      * To test special tags
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['call123TestSpecialTags'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Client, HTTP status code, HTTP response headers (array of strings)
      */
-    public function call123TestSpecialTagsWithHttpInfo($client)
+    public function call123TestSpecialTagsWithHttpInfo($client, string $contentType = self::contentTypes['call123TestSpecialTags'][0])
     {
-        $request = $this->call123TestSpecialTagsRequest($client);
+        $request = $this->call123TestSpecialTagsRequest($client, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -237,13 +246,14 @@ class AnotherFakeApi
      * To test special tags
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['call123TestSpecialTags'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function call123TestSpecialTagsAsync($client)
+    public function call123TestSpecialTagsAsync($client, string $contentType = self::contentTypes['call123TestSpecialTags'][0])
     {
-        return $this->call123TestSpecialTagsAsyncWithHttpInfo($client)
+        return $this->call123TestSpecialTagsAsyncWithHttpInfo($client, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -257,14 +267,15 @@ class AnotherFakeApi
      * To test special tags
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['call123TestSpecialTags'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function call123TestSpecialTagsAsyncWithHttpInfo($client)
+    public function call123TestSpecialTagsAsyncWithHttpInfo($client, string $contentType = self::contentTypes['call123TestSpecialTags'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Client';
-        $request = $this->call123TestSpecialTagsRequest($client);
+        $request = $this->call123TestSpecialTagsRequest($client, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -306,11 +317,12 @@ class AnotherFakeApi
      * Create request for operation 'call123TestSpecialTags'
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['call123TestSpecialTags'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function call123TestSpecialTagsRequest($client)
+    public function call123TestSpecialTagsRequest($client, string $contentType = self::contentTypes['call123TestSpecialTags'][0])
     {
 
         // verify the required parameter 'client' is set
@@ -320,6 +332,7 @@ class AnotherFakeApi
             );
         }
 
+
         $resourcePath = '/another-fake/dummy';
         $formParams = [];
         $queryParams = [];
@@ -331,20 +344,16 @@ class AnotherFakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($client)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($client));
             } else {
                 $httpBody = $client;
@@ -364,9 +373,9 @@ class AnotherFakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/DefaultApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/DefaultApi.php
index c4434410584..621e0fd0ff9 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/DefaultApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/DefaultApi.php
@@ -69,7 +69,14 @@ class DefaultApi
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [
+        'fooGet' => [
+            'application/json',
+        ],
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -118,28 +125,30 @@ class DefaultApi
     /**
      * Operation fooGet
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fooGet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\FooGetDefaultResponse
      */
-    public function fooGet()
+    public function fooGet(string $contentType = self::contentTypes['fooGet'][0])
     {
-        list($response) = $this->fooGetWithHttpInfo();
+        list($response) = $this->fooGetWithHttpInfo($contentType);
         return $response;
     }
 
     /**
      * Operation fooGetWithHttpInfo
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fooGet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\FooGetDefaultResponse, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fooGetWithHttpInfo()
+    public function fooGetWithHttpInfo(string $contentType = self::contentTypes['fooGet'][0])
     {
-        $request = $this->fooGetRequest();
+        $request = $this->fooGetRequest($contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -228,13 +237,14 @@ class DefaultApi
     /**
      * Operation fooGetAsync
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fooGet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fooGetAsync()
+    public function fooGetAsync(string $contentType = self::contentTypes['fooGet'][0])
     {
-        return $this->fooGetAsyncWithHttpInfo()
+        return $this->fooGetAsyncWithHttpInfo($contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -245,14 +255,15 @@ class DefaultApi
     /**
      * Operation fooGetAsyncWithHttpInfo
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fooGet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fooGetAsyncWithHttpInfo()
+    public function fooGetAsyncWithHttpInfo(string $contentType = self::contentTypes['fooGet'][0])
     {
         $returnType = '\OpenAPI\Client\Model\FooGetDefaultResponse';
-        $request = $this->fooGetRequest();
+        $request = $this->fooGetRequest($contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -293,13 +304,15 @@ class DefaultApi
     /**
      * Create request for operation 'fooGet'
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fooGet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fooGetRequest()
+    public function fooGetRequest(string $contentType = self::contentTypes['fooGet'][0])
     {
 
+
         $resourcePath = '/foo';
         $formParams = [];
         $queryParams = [];
@@ -311,16 +324,11 @@ class DefaultApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -338,9 +346,9 @@ class DefaultApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
index deeb2678b56..418f6d4d4e2 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
@@ -69,7 +69,63 @@ class FakeApi
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [
+        'fakeHealthGet' => [
+            'application/json',
+        ],
+        'fakeHttpSignatureTest' => [
+            'application/json',
+            'application/xml',
+        ],
+        'fakeOuterBooleanSerialize' => [
+            'application/json',
+        ],
+        'fakeOuterCompositeSerialize' => [
+            'application/json',
+        ],
+        'fakeOuterNumberSerialize' => [
+            'application/json',
+        ],
+        'fakeOuterStringSerialize' => [
+            'application/json',
+        ],
+        'fakePropertyEnumIntegerSerialize' => [
+            'application/json',
+        ],
+        'testBodyWithBinary' => [
+            'image/png',
+        ],
+        'testBodyWithFileSchema' => [
+            'application/json',
+        ],
+        'testBodyWithQueryParams' => [
+            'application/json',
+        ],
+        'testClientModel' => [
+            'application/json',
+        ],
+        'testEndpointParameters' => [
+            'application/x-www-form-urlencoded',
+        ],
+        'testEnumParameters' => [
+            'application/x-www-form-urlencoded',
+        ],
+        'testGroupParameters' => [
+            'application/json',
+        ],
+        'testInlineAdditionalProperties' => [
+            'application/json',
+        ],
+        'testJsonFormData' => [
+            'application/x-www-form-urlencoded',
+        ],
+        'testQueryParameterCollectionFormat' => [
+            'application/json',
+        ],
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -120,14 +176,15 @@ class FakeApi
      *
      * Health check endpoint
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHealthGet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\HealthCheckResult
      */
-    public function fakeHealthGet()
+    public function fakeHealthGet(string $contentType = self::contentTypes['fakeHealthGet'][0])
     {
-        list($response) = $this->fakeHealthGetWithHttpInfo();
+        list($response) = $this->fakeHealthGetWithHttpInfo($contentType);
         return $response;
     }
 
@@ -136,14 +193,15 @@ class FakeApi
      *
      * Health check endpoint
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHealthGet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\HealthCheckResult, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fakeHealthGetWithHttpInfo()
+    public function fakeHealthGetWithHttpInfo(string $contentType = self::contentTypes['fakeHealthGet'][0])
     {
-        $request = $this->fakeHealthGetRequest();
+        $request = $this->fakeHealthGetRequest($contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -234,13 +292,14 @@ class FakeApi
      *
      * Health check endpoint
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHealthGet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeHealthGetAsync()
+    public function fakeHealthGetAsync(string $contentType = self::contentTypes['fakeHealthGet'][0])
     {
-        return $this->fakeHealthGetAsyncWithHttpInfo()
+        return $this->fakeHealthGetAsyncWithHttpInfo($contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -253,14 +312,15 @@ class FakeApi
      *
      * Health check endpoint
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHealthGet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeHealthGetAsyncWithHttpInfo()
+    public function fakeHealthGetAsyncWithHttpInfo(string $contentType = self::contentTypes['fakeHealthGet'][0])
     {
         $returnType = '\OpenAPI\Client\Model\HealthCheckResult';
-        $request = $this->fakeHealthGetRequest();
+        $request = $this->fakeHealthGetRequest($contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -301,13 +361,15 @@ class FakeApi
     /**
      * Create request for operation 'fakeHealthGet'
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHealthGet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fakeHealthGetRequest()
+    public function fakeHealthGetRequest(string $contentType = self::contentTypes['fakeHealthGet'][0])
     {
 
+
         $resourcePath = '/fake/health';
         $formParams = [];
         $queryParams = [];
@@ -319,16 +381,11 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -346,9 +403,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -385,14 +442,15 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  string $query_1 query parameter (optional)
      * @param  string $header_1 header parameter (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHttpSignatureTest'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function fakeHttpSignatureTest($pet, $query_1 = null, $header_1 = null)
+    public function fakeHttpSignatureTest($pet, $query_1 = null, $header_1 = null, string $contentType = self::contentTypes['fakeHttpSignatureTest'][0])
     {
-        $this->fakeHttpSignatureTestWithHttpInfo($pet, $query_1, $header_1);
+        $this->fakeHttpSignatureTestWithHttpInfo($pet, $query_1, $header_1, $contentType);
     }
 
     /**
@@ -403,14 +461,15 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  string $query_1 query parameter (optional)
      * @param  string $header_1 header parameter (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHttpSignatureTest'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fakeHttpSignatureTestWithHttpInfo($pet, $query_1 = null, $header_1 = null)
+    public function fakeHttpSignatureTestWithHttpInfo($pet, $query_1 = null, $header_1 = null, string $contentType = self::contentTypes['fakeHttpSignatureTest'][0])
     {
-        $request = $this->fakeHttpSignatureTestRequest($pet, $query_1, $header_1);
+        $request = $this->fakeHttpSignatureTestRequest($pet, $query_1, $header_1, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -464,13 +523,14 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  string $query_1 query parameter (optional)
      * @param  string $header_1 header parameter (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHttpSignatureTest'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeHttpSignatureTestAsync($pet, $query_1 = null, $header_1 = null)
+    public function fakeHttpSignatureTestAsync($pet, $query_1 = null, $header_1 = null, string $contentType = self::contentTypes['fakeHttpSignatureTest'][0])
     {
-        return $this->fakeHttpSignatureTestAsyncWithHttpInfo($pet, $query_1, $header_1)
+        return $this->fakeHttpSignatureTestAsyncWithHttpInfo($pet, $query_1, $header_1, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -486,14 +546,15 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  string $query_1 query parameter (optional)
      * @param  string $header_1 header parameter (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHttpSignatureTest'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeHttpSignatureTestAsyncWithHttpInfo($pet, $query_1 = null, $header_1 = null)
+    public function fakeHttpSignatureTestAsyncWithHttpInfo($pet, $query_1 = null, $header_1 = null, string $contentType = self::contentTypes['fakeHttpSignatureTest'][0])
     {
         $returnType = '';
-        $request = $this->fakeHttpSignatureTestRequest($pet, $query_1, $header_1);
+        $request = $this->fakeHttpSignatureTestRequest($pet, $query_1, $header_1, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -524,11 +585,12 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  string $query_1 query parameter (optional)
      * @param  string $header_1 header parameter (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeHttpSignatureTest'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fakeHttpSignatureTestRequest($pet, $query_1 = null, $header_1 = null)
+    public function fakeHttpSignatureTestRequest($pet, $query_1 = null, $header_1 = null, string $contentType = self::contentTypes['fakeHttpSignatureTest'][0])
     {
 
         // verify the required parameter 'pet' is set
@@ -540,6 +602,7 @@ class FakeApi
 
 
 
+
         $resourcePath = '/fake/http-signature-test';
         $formParams = [];
         $queryParams = [];
@@ -564,20 +627,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json', 'application/xml']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($pet)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($pet));
             } else {
                 $httpBody = $pet;
@@ -597,9 +656,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -632,14 +691,15 @@ class FakeApi
      * Operation fakeOuterBooleanSerialize
      *
      * @param  bool $body Input boolean as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterBooleanSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return bool
      */
-    public function fakeOuterBooleanSerialize($body = null)
+    public function fakeOuterBooleanSerialize($body = null, string $contentType = self::contentTypes['fakeOuterBooleanSerialize'][0])
     {
-        list($response) = $this->fakeOuterBooleanSerializeWithHttpInfo($body);
+        list($response) = $this->fakeOuterBooleanSerializeWithHttpInfo($body, $contentType);
         return $response;
     }
 
@@ -647,14 +707,15 @@ class FakeApi
      * Operation fakeOuterBooleanSerializeWithHttpInfo
      *
      * @param  bool $body Input boolean as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterBooleanSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of bool, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fakeOuterBooleanSerializeWithHttpInfo($body = null)
+    public function fakeOuterBooleanSerializeWithHttpInfo($body = null, string $contentType = self::contentTypes['fakeOuterBooleanSerialize'][0])
     {
-        $request = $this->fakeOuterBooleanSerializeRequest($body);
+        $request = $this->fakeOuterBooleanSerializeRequest($body, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -744,13 +805,14 @@ class FakeApi
      * Operation fakeOuterBooleanSerializeAsync
      *
      * @param  bool $body Input boolean as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterBooleanSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterBooleanSerializeAsync($body = null)
+    public function fakeOuterBooleanSerializeAsync($body = null, string $contentType = self::contentTypes['fakeOuterBooleanSerialize'][0])
     {
-        return $this->fakeOuterBooleanSerializeAsyncWithHttpInfo($body)
+        return $this->fakeOuterBooleanSerializeAsyncWithHttpInfo($body, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -762,14 +824,15 @@ class FakeApi
      * Operation fakeOuterBooleanSerializeAsyncWithHttpInfo
      *
      * @param  bool $body Input boolean as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterBooleanSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterBooleanSerializeAsyncWithHttpInfo($body = null)
+    public function fakeOuterBooleanSerializeAsyncWithHttpInfo($body = null, string $contentType = self::contentTypes['fakeOuterBooleanSerialize'][0])
     {
         $returnType = 'bool';
-        $request = $this->fakeOuterBooleanSerializeRequest($body);
+        $request = $this->fakeOuterBooleanSerializeRequest($body, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -811,14 +874,16 @@ class FakeApi
      * Create request for operation 'fakeOuterBooleanSerialize'
      *
      * @param  bool $body Input boolean as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterBooleanSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fakeOuterBooleanSerializeRequest($body = null)
+    public function fakeOuterBooleanSerializeRequest($body = null, string $contentType = self::contentTypes['fakeOuterBooleanSerialize'][0])
     {
 
 
+
         $resourcePath = '/fake/outer/boolean';
         $formParams = [];
         $queryParams = [];
@@ -830,20 +895,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['*/*']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['*/*'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['*/*', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($body)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($body));
             } else {
                 $httpBody = $body;
@@ -863,9 +924,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -898,14 +959,15 @@ class FakeApi
      * Operation fakeOuterCompositeSerialize
      *
      * @param  \OpenAPI\Client\Model\OuterComposite $outer_composite Input composite as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterCompositeSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\OuterComposite
      */
-    public function fakeOuterCompositeSerialize($outer_composite = null)
+    public function fakeOuterCompositeSerialize($outer_composite = null, string $contentType = self::contentTypes['fakeOuterCompositeSerialize'][0])
     {
-        list($response) = $this->fakeOuterCompositeSerializeWithHttpInfo($outer_composite);
+        list($response) = $this->fakeOuterCompositeSerializeWithHttpInfo($outer_composite, $contentType);
         return $response;
     }
 
@@ -913,14 +975,15 @@ class FakeApi
      * Operation fakeOuterCompositeSerializeWithHttpInfo
      *
      * @param  \OpenAPI\Client\Model\OuterComposite $outer_composite Input composite as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterCompositeSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\OuterComposite, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fakeOuterCompositeSerializeWithHttpInfo($outer_composite = null)
+    public function fakeOuterCompositeSerializeWithHttpInfo($outer_composite = null, string $contentType = self::contentTypes['fakeOuterCompositeSerialize'][0])
     {
-        $request = $this->fakeOuterCompositeSerializeRequest($outer_composite);
+        $request = $this->fakeOuterCompositeSerializeRequest($outer_composite, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1010,13 +1073,14 @@ class FakeApi
      * Operation fakeOuterCompositeSerializeAsync
      *
      * @param  \OpenAPI\Client\Model\OuterComposite $outer_composite Input composite as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterCompositeSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterCompositeSerializeAsync($outer_composite = null)
+    public function fakeOuterCompositeSerializeAsync($outer_composite = null, string $contentType = self::contentTypes['fakeOuterCompositeSerialize'][0])
     {
-        return $this->fakeOuterCompositeSerializeAsyncWithHttpInfo($outer_composite)
+        return $this->fakeOuterCompositeSerializeAsyncWithHttpInfo($outer_composite, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1028,14 +1092,15 @@ class FakeApi
      * Operation fakeOuterCompositeSerializeAsyncWithHttpInfo
      *
      * @param  \OpenAPI\Client\Model\OuterComposite $outer_composite Input composite as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterCompositeSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterCompositeSerializeAsyncWithHttpInfo($outer_composite = null)
+    public function fakeOuterCompositeSerializeAsyncWithHttpInfo($outer_composite = null, string $contentType = self::contentTypes['fakeOuterCompositeSerialize'][0])
     {
         $returnType = '\OpenAPI\Client\Model\OuterComposite';
-        $request = $this->fakeOuterCompositeSerializeRequest($outer_composite);
+        $request = $this->fakeOuterCompositeSerializeRequest($outer_composite, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1077,14 +1142,16 @@ class FakeApi
      * Create request for operation 'fakeOuterCompositeSerialize'
      *
      * @param  \OpenAPI\Client\Model\OuterComposite $outer_composite Input composite as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterCompositeSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fakeOuterCompositeSerializeRequest($outer_composite = null)
+    public function fakeOuterCompositeSerializeRequest($outer_composite = null, string $contentType = self::contentTypes['fakeOuterCompositeSerialize'][0])
     {
 
 
+
         $resourcePath = '/fake/outer/composite';
         $formParams = [];
         $queryParams = [];
@@ -1096,20 +1163,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['*/*']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['*/*'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['*/*', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($outer_composite)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($outer_composite));
             } else {
                 $httpBody = $outer_composite;
@@ -1129,9 +1192,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1164,14 +1227,15 @@ class FakeApi
      * Operation fakeOuterNumberSerialize
      *
      * @param  float $body Input number as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterNumberSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return float
      */
-    public function fakeOuterNumberSerialize($body = null)
+    public function fakeOuterNumberSerialize($body = null, string $contentType = self::contentTypes['fakeOuterNumberSerialize'][0])
     {
-        list($response) = $this->fakeOuterNumberSerializeWithHttpInfo($body);
+        list($response) = $this->fakeOuterNumberSerializeWithHttpInfo($body, $contentType);
         return $response;
     }
 
@@ -1179,14 +1243,15 @@ class FakeApi
      * Operation fakeOuterNumberSerializeWithHttpInfo
      *
      * @param  float $body Input number as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterNumberSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of float, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fakeOuterNumberSerializeWithHttpInfo($body = null)
+    public function fakeOuterNumberSerializeWithHttpInfo($body = null, string $contentType = self::contentTypes['fakeOuterNumberSerialize'][0])
     {
-        $request = $this->fakeOuterNumberSerializeRequest($body);
+        $request = $this->fakeOuterNumberSerializeRequest($body, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1276,13 +1341,14 @@ class FakeApi
      * Operation fakeOuterNumberSerializeAsync
      *
      * @param  float $body Input number as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterNumberSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterNumberSerializeAsync($body = null)
+    public function fakeOuterNumberSerializeAsync($body = null, string $contentType = self::contentTypes['fakeOuterNumberSerialize'][0])
     {
-        return $this->fakeOuterNumberSerializeAsyncWithHttpInfo($body)
+        return $this->fakeOuterNumberSerializeAsyncWithHttpInfo($body, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1294,14 +1360,15 @@ class FakeApi
      * Operation fakeOuterNumberSerializeAsyncWithHttpInfo
      *
      * @param  float $body Input number as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterNumberSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterNumberSerializeAsyncWithHttpInfo($body = null)
+    public function fakeOuterNumberSerializeAsyncWithHttpInfo($body = null, string $contentType = self::contentTypes['fakeOuterNumberSerialize'][0])
     {
         $returnType = 'float';
-        $request = $this->fakeOuterNumberSerializeRequest($body);
+        $request = $this->fakeOuterNumberSerializeRequest($body, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1343,14 +1410,16 @@ class FakeApi
      * Create request for operation 'fakeOuterNumberSerialize'
      *
      * @param  float $body Input number as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterNumberSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fakeOuterNumberSerializeRequest($body = null)
+    public function fakeOuterNumberSerializeRequest($body = null, string $contentType = self::contentTypes['fakeOuterNumberSerialize'][0])
     {
 
 
+
         $resourcePath = '/fake/outer/number';
         $formParams = [];
         $queryParams = [];
@@ -1362,20 +1431,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['*/*']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['*/*'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['*/*', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($body)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($body));
             } else {
                 $httpBody = $body;
@@ -1395,9 +1460,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1430,14 +1495,15 @@ class FakeApi
      * Operation fakeOuterStringSerialize
      *
      * @param  string $body Input string as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterStringSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return string
      */
-    public function fakeOuterStringSerialize($body = null)
+    public function fakeOuterStringSerialize($body = null, string $contentType = self::contentTypes['fakeOuterStringSerialize'][0])
     {
-        list($response) = $this->fakeOuterStringSerializeWithHttpInfo($body);
+        list($response) = $this->fakeOuterStringSerializeWithHttpInfo($body, $contentType);
         return $response;
     }
 
@@ -1445,14 +1511,15 @@ class FakeApi
      * Operation fakeOuterStringSerializeWithHttpInfo
      *
      * @param  string $body Input string as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterStringSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of string, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fakeOuterStringSerializeWithHttpInfo($body = null)
+    public function fakeOuterStringSerializeWithHttpInfo($body = null, string $contentType = self::contentTypes['fakeOuterStringSerialize'][0])
     {
-        $request = $this->fakeOuterStringSerializeRequest($body);
+        $request = $this->fakeOuterStringSerializeRequest($body, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1542,13 +1609,14 @@ class FakeApi
      * Operation fakeOuterStringSerializeAsync
      *
      * @param  string $body Input string as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterStringSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterStringSerializeAsync($body = null)
+    public function fakeOuterStringSerializeAsync($body = null, string $contentType = self::contentTypes['fakeOuterStringSerialize'][0])
     {
-        return $this->fakeOuterStringSerializeAsyncWithHttpInfo($body)
+        return $this->fakeOuterStringSerializeAsyncWithHttpInfo($body, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1560,14 +1628,15 @@ class FakeApi
      * Operation fakeOuterStringSerializeAsyncWithHttpInfo
      *
      * @param  string $body Input string as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterStringSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakeOuterStringSerializeAsyncWithHttpInfo($body = null)
+    public function fakeOuterStringSerializeAsyncWithHttpInfo($body = null, string $contentType = self::contentTypes['fakeOuterStringSerialize'][0])
     {
         $returnType = 'string';
-        $request = $this->fakeOuterStringSerializeRequest($body);
+        $request = $this->fakeOuterStringSerializeRequest($body, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1609,14 +1678,16 @@ class FakeApi
      * Create request for operation 'fakeOuterStringSerialize'
      *
      * @param  string $body Input string as post body (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakeOuterStringSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fakeOuterStringSerializeRequest($body = null)
+    public function fakeOuterStringSerializeRequest($body = null, string $contentType = self::contentTypes['fakeOuterStringSerialize'][0])
     {
 
 
+
         $resourcePath = '/fake/outer/string';
         $formParams = [];
         $queryParams = [];
@@ -1628,20 +1699,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['*/*']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['*/*'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['*/*', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($body)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($body));
             } else {
                 $httpBody = $body;
@@ -1661,9 +1728,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1696,14 +1763,15 @@ class FakeApi
      * Operation fakePropertyEnumIntegerSerialize
      *
      * @param  \OpenAPI\Client\Model\OuterObjectWithEnumProperty $outer_object_with_enum_property Input enum (int) as post body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakePropertyEnumIntegerSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\OuterObjectWithEnumProperty
      */
-    public function fakePropertyEnumIntegerSerialize($outer_object_with_enum_property)
+    public function fakePropertyEnumIntegerSerialize($outer_object_with_enum_property, string $contentType = self::contentTypes['fakePropertyEnumIntegerSerialize'][0])
     {
-        list($response) = $this->fakePropertyEnumIntegerSerializeWithHttpInfo($outer_object_with_enum_property);
+        list($response) = $this->fakePropertyEnumIntegerSerializeWithHttpInfo($outer_object_with_enum_property, $contentType);
         return $response;
     }
 
@@ -1711,14 +1779,15 @@ class FakeApi
      * Operation fakePropertyEnumIntegerSerializeWithHttpInfo
      *
      * @param  \OpenAPI\Client\Model\OuterObjectWithEnumProperty $outer_object_with_enum_property Input enum (int) as post body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakePropertyEnumIntegerSerialize'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\OuterObjectWithEnumProperty, HTTP status code, HTTP response headers (array of strings)
      */
-    public function fakePropertyEnumIntegerSerializeWithHttpInfo($outer_object_with_enum_property)
+    public function fakePropertyEnumIntegerSerializeWithHttpInfo($outer_object_with_enum_property, string $contentType = self::contentTypes['fakePropertyEnumIntegerSerialize'][0])
     {
-        $request = $this->fakePropertyEnumIntegerSerializeRequest($outer_object_with_enum_property);
+        $request = $this->fakePropertyEnumIntegerSerializeRequest($outer_object_with_enum_property, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1808,13 +1877,14 @@ class FakeApi
      * Operation fakePropertyEnumIntegerSerializeAsync
      *
      * @param  \OpenAPI\Client\Model\OuterObjectWithEnumProperty $outer_object_with_enum_property Input enum (int) as post body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakePropertyEnumIntegerSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakePropertyEnumIntegerSerializeAsync($outer_object_with_enum_property)
+    public function fakePropertyEnumIntegerSerializeAsync($outer_object_with_enum_property, string $contentType = self::contentTypes['fakePropertyEnumIntegerSerialize'][0])
     {
-        return $this->fakePropertyEnumIntegerSerializeAsyncWithHttpInfo($outer_object_with_enum_property)
+        return $this->fakePropertyEnumIntegerSerializeAsyncWithHttpInfo($outer_object_with_enum_property, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1826,14 +1896,15 @@ class FakeApi
      * Operation fakePropertyEnumIntegerSerializeAsyncWithHttpInfo
      *
      * @param  \OpenAPI\Client\Model\OuterObjectWithEnumProperty $outer_object_with_enum_property Input enum (int) as post body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakePropertyEnumIntegerSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function fakePropertyEnumIntegerSerializeAsyncWithHttpInfo($outer_object_with_enum_property)
+    public function fakePropertyEnumIntegerSerializeAsyncWithHttpInfo($outer_object_with_enum_property, string $contentType = self::contentTypes['fakePropertyEnumIntegerSerialize'][0])
     {
         $returnType = '\OpenAPI\Client\Model\OuterObjectWithEnumProperty';
-        $request = $this->fakePropertyEnumIntegerSerializeRequest($outer_object_with_enum_property);
+        $request = $this->fakePropertyEnumIntegerSerializeRequest($outer_object_with_enum_property, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1875,11 +1946,12 @@ class FakeApi
      * Create request for operation 'fakePropertyEnumIntegerSerialize'
      *
      * @param  \OpenAPI\Client\Model\OuterObjectWithEnumProperty $outer_object_with_enum_property Input enum (int) as post body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['fakePropertyEnumIntegerSerialize'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function fakePropertyEnumIntegerSerializeRequest($outer_object_with_enum_property)
+    public function fakePropertyEnumIntegerSerializeRequest($outer_object_with_enum_property, string $contentType = self::contentTypes['fakePropertyEnumIntegerSerialize'][0])
     {
 
         // verify the required parameter 'outer_object_with_enum_property' is set
@@ -1889,6 +1961,7 @@ class FakeApi
             );
         }
 
+
         $resourcePath = '/fake/property/enum-int';
         $formParams = [];
         $queryParams = [];
@@ -1900,20 +1973,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['*/*']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['*/*'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['*/*', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($outer_object_with_enum_property)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($outer_object_with_enum_property));
             } else {
                 $httpBody = $outer_object_with_enum_property;
@@ -1933,9 +2002,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1968,28 +2037,30 @@ class FakeApi
      * Operation testBodyWithBinary
      *
      * @param  \SplFileObject $body image to upload (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithBinary'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testBodyWithBinary($body)
+    public function testBodyWithBinary($body, string $contentType = self::contentTypes['testBodyWithBinary'][0])
     {
-        $this->testBodyWithBinaryWithHttpInfo($body);
+        $this->testBodyWithBinaryWithHttpInfo($body, $contentType);
     }
 
     /**
      * Operation testBodyWithBinaryWithHttpInfo
      *
      * @param  \SplFileObject $body image to upload (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithBinary'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testBodyWithBinaryWithHttpInfo($body)
+    public function testBodyWithBinaryWithHttpInfo($body, string $contentType = self::contentTypes['testBodyWithBinary'][0])
     {
-        $request = $this->testBodyWithBinaryRequest($body);
+        $request = $this->testBodyWithBinaryRequest($body, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -2039,13 +2110,14 @@ class FakeApi
      * Operation testBodyWithBinaryAsync
      *
      * @param  \SplFileObject $body image to upload (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithBinary'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testBodyWithBinaryAsync($body)
+    public function testBodyWithBinaryAsync($body, string $contentType = self::contentTypes['testBodyWithBinary'][0])
     {
-        return $this->testBodyWithBinaryAsyncWithHttpInfo($body)
+        return $this->testBodyWithBinaryAsyncWithHttpInfo($body, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -2057,14 +2129,15 @@ class FakeApi
      * Operation testBodyWithBinaryAsyncWithHttpInfo
      *
      * @param  \SplFileObject $body image to upload (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithBinary'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testBodyWithBinaryAsyncWithHttpInfo($body)
+    public function testBodyWithBinaryAsyncWithHttpInfo($body, string $contentType = self::contentTypes['testBodyWithBinary'][0])
     {
         $returnType = '';
-        $request = $this->testBodyWithBinaryRequest($body);
+        $request = $this->testBodyWithBinaryRequest($body, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -2093,11 +2166,12 @@ class FakeApi
      * Create request for operation 'testBodyWithBinary'
      *
      * @param  \SplFileObject $body image to upload (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithBinary'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testBodyWithBinaryRequest($body)
+    public function testBodyWithBinaryRequest($body, string $contentType = self::contentTypes['testBodyWithBinary'][0])
     {
 
         // verify the required parameter 'body' is set
@@ -2107,6 +2181,7 @@ class FakeApi
             );
         }
 
+
         $resourcePath = '/fake/body-with-binary';
         $formParams = [];
         $queryParams = [];
@@ -2118,20 +2193,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['image/png']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($body)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($body));
             } else {
                 $httpBody = $body;
@@ -2151,9 +2222,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -2186,28 +2257,30 @@ class FakeApi
      * Operation testBodyWithFileSchema
      *
      * @param  \OpenAPI\Client\Model\FileSchemaTestClass $file_schema_test_class file_schema_test_class (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithFileSchema'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testBodyWithFileSchema($file_schema_test_class)
+    public function testBodyWithFileSchema($file_schema_test_class, string $contentType = self::contentTypes['testBodyWithFileSchema'][0])
     {
-        $this->testBodyWithFileSchemaWithHttpInfo($file_schema_test_class);
+        $this->testBodyWithFileSchemaWithHttpInfo($file_schema_test_class, $contentType);
     }
 
     /**
      * Operation testBodyWithFileSchemaWithHttpInfo
      *
      * @param  \OpenAPI\Client\Model\FileSchemaTestClass $file_schema_test_class (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithFileSchema'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testBodyWithFileSchemaWithHttpInfo($file_schema_test_class)
+    public function testBodyWithFileSchemaWithHttpInfo($file_schema_test_class, string $contentType = self::contentTypes['testBodyWithFileSchema'][0])
     {
-        $request = $this->testBodyWithFileSchemaRequest($file_schema_test_class);
+        $request = $this->testBodyWithFileSchemaRequest($file_schema_test_class, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -2257,13 +2330,14 @@ class FakeApi
      * Operation testBodyWithFileSchemaAsync
      *
      * @param  \OpenAPI\Client\Model\FileSchemaTestClass $file_schema_test_class (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithFileSchema'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testBodyWithFileSchemaAsync($file_schema_test_class)
+    public function testBodyWithFileSchemaAsync($file_schema_test_class, string $contentType = self::contentTypes['testBodyWithFileSchema'][0])
     {
-        return $this->testBodyWithFileSchemaAsyncWithHttpInfo($file_schema_test_class)
+        return $this->testBodyWithFileSchemaAsyncWithHttpInfo($file_schema_test_class, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -2275,14 +2349,15 @@ class FakeApi
      * Operation testBodyWithFileSchemaAsyncWithHttpInfo
      *
      * @param  \OpenAPI\Client\Model\FileSchemaTestClass $file_schema_test_class (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithFileSchema'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testBodyWithFileSchemaAsyncWithHttpInfo($file_schema_test_class)
+    public function testBodyWithFileSchemaAsyncWithHttpInfo($file_schema_test_class, string $contentType = self::contentTypes['testBodyWithFileSchema'][0])
     {
         $returnType = '';
-        $request = $this->testBodyWithFileSchemaRequest($file_schema_test_class);
+        $request = $this->testBodyWithFileSchemaRequest($file_schema_test_class, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -2311,11 +2386,12 @@ class FakeApi
      * Create request for operation 'testBodyWithFileSchema'
      *
      * @param  \OpenAPI\Client\Model\FileSchemaTestClass $file_schema_test_class (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithFileSchema'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testBodyWithFileSchemaRequest($file_schema_test_class)
+    public function testBodyWithFileSchemaRequest($file_schema_test_class, string $contentType = self::contentTypes['testBodyWithFileSchema'][0])
     {
 
         // verify the required parameter 'file_schema_test_class' is set
@@ -2325,6 +2401,7 @@ class FakeApi
             );
         }
 
+
         $resourcePath = '/fake/body-with-file-schema';
         $formParams = [];
         $queryParams = [];
@@ -2336,20 +2413,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($file_schema_test_class)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($file_schema_test_class));
             } else {
                 $httpBody = $file_schema_test_class;
@@ -2369,9 +2442,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -2405,14 +2478,15 @@ class FakeApi
      *
      * @param  string $query query (required)
      * @param  \OpenAPI\Client\Model\User $user user (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithQueryParams'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testBodyWithQueryParams($query, $user)
+    public function testBodyWithQueryParams($query, $user, string $contentType = self::contentTypes['testBodyWithQueryParams'][0])
     {
-        $this->testBodyWithQueryParamsWithHttpInfo($query, $user);
+        $this->testBodyWithQueryParamsWithHttpInfo($query, $user, $contentType);
     }
 
     /**
@@ -2420,14 +2494,15 @@ class FakeApi
      *
      * @param  string $query (required)
      * @param  \OpenAPI\Client\Model\User $user (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithQueryParams'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testBodyWithQueryParamsWithHttpInfo($query, $user)
+    public function testBodyWithQueryParamsWithHttpInfo($query, $user, string $contentType = self::contentTypes['testBodyWithQueryParams'][0])
     {
-        $request = $this->testBodyWithQueryParamsRequest($query, $user);
+        $request = $this->testBodyWithQueryParamsRequest($query, $user, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -2478,13 +2553,14 @@ class FakeApi
      *
      * @param  string $query (required)
      * @param  \OpenAPI\Client\Model\User $user (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithQueryParams'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testBodyWithQueryParamsAsync($query, $user)
+    public function testBodyWithQueryParamsAsync($query, $user, string $contentType = self::contentTypes['testBodyWithQueryParams'][0])
     {
-        return $this->testBodyWithQueryParamsAsyncWithHttpInfo($query, $user)
+        return $this->testBodyWithQueryParamsAsyncWithHttpInfo($query, $user, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -2497,14 +2573,15 @@ class FakeApi
      *
      * @param  string $query (required)
      * @param  \OpenAPI\Client\Model\User $user (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithQueryParams'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testBodyWithQueryParamsAsyncWithHttpInfo($query, $user)
+    public function testBodyWithQueryParamsAsyncWithHttpInfo($query, $user, string $contentType = self::contentTypes['testBodyWithQueryParams'][0])
     {
         $returnType = '';
-        $request = $this->testBodyWithQueryParamsRequest($query, $user);
+        $request = $this->testBodyWithQueryParamsRequest($query, $user, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -2534,11 +2611,12 @@ class FakeApi
      *
      * @param  string $query (required)
      * @param  \OpenAPI\Client\Model\User $user (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testBodyWithQueryParams'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testBodyWithQueryParamsRequest($query, $user)
+    public function testBodyWithQueryParamsRequest($query, $user, string $contentType = self::contentTypes['testBodyWithQueryParams'][0])
     {
 
         // verify the required parameter 'query' is set
@@ -2555,6 +2633,7 @@ class FakeApi
             );
         }
 
+
         $resourcePath = '/fake/body-with-query-params';
         $formParams = [];
         $queryParams = [];
@@ -2575,20 +2654,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($user)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($user));
             } else {
                 $httpBody = $user;
@@ -2608,9 +2683,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -2645,14 +2720,15 @@ class FakeApi
      * To test \&quot;client\&quot; model
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClientModel'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Client
      */
-    public function testClientModel($client)
+    public function testClientModel($client, string $contentType = self::contentTypes['testClientModel'][0])
     {
-        list($response) = $this->testClientModelWithHttpInfo($client);
+        list($response) = $this->testClientModelWithHttpInfo($client, $contentType);
         return $response;
     }
 
@@ -2662,14 +2738,15 @@ class FakeApi
      * To test \&quot;client\&quot; model
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClientModel'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Client, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testClientModelWithHttpInfo($client)
+    public function testClientModelWithHttpInfo($client, string $contentType = self::contentTypes['testClientModel'][0])
     {
-        $request = $this->testClientModelRequest($client);
+        $request = $this->testClientModelRequest($client, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -2761,13 +2838,14 @@ class FakeApi
      * To test \&quot;client\&quot; model
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClientModel'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testClientModelAsync($client)
+    public function testClientModelAsync($client, string $contentType = self::contentTypes['testClientModel'][0])
     {
-        return $this->testClientModelAsyncWithHttpInfo($client)
+        return $this->testClientModelAsyncWithHttpInfo($client, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -2781,14 +2859,15 @@ class FakeApi
      * To test \&quot;client\&quot; model
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClientModel'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testClientModelAsyncWithHttpInfo($client)
+    public function testClientModelAsyncWithHttpInfo($client, string $contentType = self::contentTypes['testClientModel'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Client';
-        $request = $this->testClientModelRequest($client);
+        $request = $this->testClientModelRequest($client, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -2830,11 +2909,12 @@ class FakeApi
      * Create request for operation 'testClientModel'
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClientModel'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testClientModelRequest($client)
+    public function testClientModelRequest($client, string $contentType = self::contentTypes['testClientModel'][0])
     {
 
         // verify the required parameter 'client' is set
@@ -2844,6 +2924,7 @@ class FakeApi
             );
         }
 
+
         $resourcePath = '/fake';
         $formParams = [];
         $queryParams = [];
@@ -2855,20 +2936,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($client)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($client));
             } else {
                 $httpBody = $client;
@@ -2888,9 +2965,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -2938,14 +3015,15 @@ class FakeApi
      * @param  \DateTime $date_time None (optional)
      * @param  string $password None (optional)
      * @param  string $callback None (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEndpointParameters'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testEndpointParameters($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null)
+    public function testEndpointParameters($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null, string $contentType = self::contentTypes['testEndpointParameters'][0])
     {
-        $this->testEndpointParametersWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback);
+        $this->testEndpointParametersWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback, $contentType);
     }
 
     /**
@@ -2967,14 +3045,15 @@ class FakeApi
      * @param  \DateTime $date_time None (optional)
      * @param  string $password None (optional)
      * @param  string $callback None (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEndpointParameters'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testEndpointParametersWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null)
+    public function testEndpointParametersWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null, string $contentType = self::contentTypes['testEndpointParameters'][0])
     {
-        $request = $this->testEndpointParametersRequest($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback);
+        $request = $this->testEndpointParametersRequest($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -3039,13 +3118,14 @@ class FakeApi
      * @param  \DateTime $date_time None (optional)
      * @param  string $password None (optional)
      * @param  string $callback None (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEndpointParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testEndpointParametersAsync($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null)
+    public function testEndpointParametersAsync($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null, string $contentType = self::contentTypes['testEndpointParameters'][0])
     {
-        return $this->testEndpointParametersAsyncWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback)
+        return $this->testEndpointParametersAsyncWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -3072,14 +3152,15 @@ class FakeApi
      * @param  \DateTime $date_time None (optional)
      * @param  string $password None (optional)
      * @param  string $callback None (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEndpointParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testEndpointParametersAsyncWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null)
+    public function testEndpointParametersAsyncWithHttpInfo($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null, string $contentType = self::contentTypes['testEndpointParameters'][0])
     {
         $returnType = '';
-        $request = $this->testEndpointParametersRequest($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback);
+        $request = $this->testEndpointParametersRequest($number, $double, $pattern_without_delimiter, $byte, $integer, $int32, $int64, $float, $string, $binary, $date, $date_time, $password, $callback, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -3121,11 +3202,12 @@ class FakeApi
      * @param  \DateTime $date_time None (optional)
      * @param  string $password None (optional)
      * @param  string $callback None (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEndpointParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testEndpointParametersRequest($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null)
+    public function testEndpointParametersRequest($number, $double, $pattern_without_delimiter, $byte, $integer = null, $int32 = null, $int64 = null, $float = null, $string = null, $binary = null, $date = null, $date_time = null, $password = null, $callback = null, string $contentType = self::contentTypes['testEndpointParameters'][0])
     {
 
         // verify the required parameter 'number' is set
@@ -3140,8 +3222,7 @@ class FakeApi
         if ($number < 32.1) {
             throw new \InvalidArgumentException('invalid value for "$number" when calling FakeApi.testEndpointParameters, must be bigger than or equal to 32.1.');
         }
-
-
+        
         // verify the required parameter 'double' is set
         if ($double === null || (is_array($double) && count($double) === 0)) {
             throw new \InvalidArgumentException(
@@ -3154,8 +3235,7 @@ class FakeApi
         if ($double < 67.8) {
             throw new \InvalidArgumentException('invalid value for "$double" when calling FakeApi.testEndpointParameters, must be bigger than or equal to 67.8.');
         }
-
-
+        
         // verify the required parameter 'pattern_without_delimiter' is set
         if ($pattern_without_delimiter === null || (is_array($pattern_without_delimiter) && count($pattern_without_delimiter) === 0)) {
             throw new \InvalidArgumentException(
@@ -3165,8 +3245,7 @@ class FakeApi
         if (!preg_match("/^[A-Z].*/", $pattern_without_delimiter)) {
             throw new \InvalidArgumentException("invalid value for \"pattern_without_delimiter\" when calling FakeApi.testEndpointParameters, must conform to the pattern /^[A-Z].*/.");
         }
-
-
+        
         // verify the required parameter 'byte' is set
         if ($byte === null || (is_array($byte) && count($byte) === 0)) {
             throw new \InvalidArgumentException(
@@ -3180,27 +3259,23 @@ class FakeApi
         if ($integer !== null && $integer < 10) {
             throw new \InvalidArgumentException('invalid value for "$integer" when calling FakeApi.testEndpointParameters, must be bigger than or equal to 10.');
         }
-
-
+        
         if ($int32 !== null && $int32 > 200) {
             throw new \InvalidArgumentException('invalid value for "$int32" when calling FakeApi.testEndpointParameters, must be smaller than or equal to 200.');
         }
         if ($int32 !== null && $int32 < 20) {
             throw new \InvalidArgumentException('invalid value for "$int32" when calling FakeApi.testEndpointParameters, must be bigger than or equal to 20.');
         }
-
-
+        
 
         if ($float !== null && $float > 987.6) {
             throw new \InvalidArgumentException('invalid value for "$float" when calling FakeApi.testEndpointParameters, must be smaller than or equal to 987.6.');
         }
-
-
+        
         if ($string !== null && !preg_match("/[a-z]/i", $string)) {
             throw new \InvalidArgumentException("invalid value for \"string\" when calling FakeApi.testEndpointParameters, must conform to the pattern /[a-z]/i.");
         }
-
-
+        
 
 
 
@@ -3210,7 +3285,7 @@ class FakeApi
         if ($password !== null && strlen($password) < 10) {
             throw new \InvalidArgumentException('invalid length for "$password" when calling FakeApi.testEndpointParameters, must be bigger than or equal to 10.');
         }
-
+        
 
 
         $resourcePath = '/fake';
@@ -3288,16 +3363,11 @@ class FakeApi
             $formParams['callback'] = ObjectSerializer::toFormValue($callback);
         }
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/x-www-form-urlencoded']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -3315,9 +3385,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -3364,14 +3434,15 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\EnumClass[] $enum_query_model_array enum_query_model_array (optional)
      * @param  string[] $enum_form_string_array Form parameter enum test (string array) (optional, default to '$')
      * @param  string $enum_form_string Form parameter enum test (string) (optional, default to '-efg')
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEnumParameters'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testEnumParameters($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg')
+    public function testEnumParameters($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg', string $contentType = self::contentTypes['testEnumParameters'][0])
     {
-        $this->testEnumParametersWithHttpInfo($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string);
+        $this->testEnumParametersWithHttpInfo($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string, $contentType);
     }
 
     /**
@@ -3388,14 +3459,15 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\EnumClass[] $enum_query_model_array (optional)
      * @param  string[] $enum_form_string_array Form parameter enum test (string array) (optional, default to '$')
      * @param  string $enum_form_string Form parameter enum test (string) (optional, default to '-efg')
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEnumParameters'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testEnumParametersWithHttpInfo($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg')
+    public function testEnumParametersWithHttpInfo($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg', string $contentType = self::contentTypes['testEnumParameters'][0])
     {
-        $request = $this->testEnumParametersRequest($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string);
+        $request = $this->testEnumParametersRequest($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -3455,13 +3527,14 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\EnumClass[] $enum_query_model_array (optional)
      * @param  string[] $enum_form_string_array Form parameter enum test (string array) (optional, default to '$')
      * @param  string $enum_form_string Form parameter enum test (string) (optional, default to '-efg')
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEnumParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testEnumParametersAsync($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg')
+    public function testEnumParametersAsync($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg', string $contentType = self::contentTypes['testEnumParameters'][0])
     {
-        return $this->testEnumParametersAsyncWithHttpInfo($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string)
+        return $this->testEnumParametersAsyncWithHttpInfo($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -3483,14 +3556,15 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\EnumClass[] $enum_query_model_array (optional)
      * @param  string[] $enum_form_string_array Form parameter enum test (string array) (optional, default to '$')
      * @param  string $enum_form_string Form parameter enum test (string) (optional, default to '-efg')
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEnumParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testEnumParametersAsyncWithHttpInfo($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg')
+    public function testEnumParametersAsyncWithHttpInfo($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg', string $contentType = self::contentTypes['testEnumParameters'][0])
     {
         $returnType = '';
-        $request = $this->testEnumParametersRequest($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string);
+        $request = $this->testEnumParametersRequest($enum_header_string_array, $enum_header_string, $enum_query_string_array, $enum_query_string, $enum_query_integer, $enum_query_double, $enum_query_model_array, $enum_form_string_array, $enum_form_string, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -3527,11 +3601,12 @@ class FakeApi
      * @param  \OpenAPI\Client\Model\EnumClass[] $enum_query_model_array (optional)
      * @param  string[] $enum_form_string_array Form parameter enum test (string array) (optional, default to '$')
      * @param  string $enum_form_string Form parameter enum test (string) (optional, default to '-efg')
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testEnumParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testEnumParametersRequest($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg')
+    public function testEnumParametersRequest($enum_header_string_array = null, $enum_header_string = '-efg', $enum_query_string_array = null, $enum_query_string = '-efg', $enum_query_integer = null, $enum_query_double = null, $enum_query_model_array = null, $enum_form_string_array = '$', $enum_form_string = '-efg', string $contentType = self::contentTypes['testEnumParameters'][0])
     {
 
 
@@ -3543,6 +3618,7 @@ class FakeApi
 
 
 
+
         $resourcePath = '/fake';
         $formParams = [];
         $queryParams = [];
@@ -3618,16 +3694,11 @@ class FakeApi
             $formParams['enum_form_string'] = ObjectSerializer::toFormValue($enum_form_string);
         }
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/x-www-form-urlencoded']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -3645,9 +3716,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -3689,6 +3760,7 @@ class FakeApi
      * @param  int $string_group String in group parameters (optional)
      * @param  bool $boolean_group Boolean in group parameters (optional)
      * @param  int $int64_group Integer in group parameters (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testGroupParameters'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
@@ -3712,6 +3784,7 @@ class FakeApi
      * @param  int $string_group String in group parameters (optional)
      * @param  bool $boolean_group Boolean in group parameters (optional)
      * @param  int $int64_group Integer in group parameters (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testGroupParameters'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
@@ -3778,6 +3851,7 @@ class FakeApi
      * @param  int $string_group String in group parameters (optional)
      * @param  bool $boolean_group Boolean in group parameters (optional)
      * @param  int $int64_group Integer in group parameters (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testGroupParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
@@ -3805,6 +3879,7 @@ class FakeApi
      * @param  int $string_group String in group parameters (optional)
      * @param  bool $boolean_group Boolean in group parameters (optional)
      * @param  int $int64_group Integer in group parameters (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testGroupParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
@@ -3848,6 +3923,7 @@ class FakeApi
      * @param  int $string_group String in group parameters (optional)
      * @param  bool $boolean_group Boolean in group parameters (optional)
      * @param  int $int64_group Integer in group parameters (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testGroupParameters'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
@@ -3861,7 +3937,8 @@ class FakeApi
         $string_group = array_key_exists('string_group', $associative_array) ? $associative_array['string_group'] : null;
         $boolean_group = array_key_exists('boolean_group', $associative_array) ? $associative_array['boolean_group'] : null;
         $int64_group = array_key_exists('int64_group', $associative_array) ? $associative_array['int64_group'] : null;
-
+        $contentType = $associative_array['contentType'] ?? self::contentTypes['testGroupParameters'][0];
+        
         // verify the required parameter 'required_string_group' is set
         if ($required_string_group === null || (is_array($required_string_group) && count($required_string_group) === 0)) {
             throw new \InvalidArgumentException(
@@ -3886,6 +3963,7 @@ class FakeApi
 
 
 
+
         $resourcePath = '/fake';
         $formParams = [];
         $queryParams = [];
@@ -3941,16 +4019,11 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -3968,9 +4041,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -4009,14 +4082,15 @@ class FakeApi
      * test inline additionalProperties
      *
      * @param  array<string,string> $request_body request body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testInlineAdditionalProperties'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testInlineAdditionalProperties($request_body)
+    public function testInlineAdditionalProperties($request_body, string $contentType = self::contentTypes['testInlineAdditionalProperties'][0])
     {
-        $this->testInlineAdditionalPropertiesWithHttpInfo($request_body);
+        $this->testInlineAdditionalPropertiesWithHttpInfo($request_body, $contentType);
     }
 
     /**
@@ -4025,14 +4099,15 @@ class FakeApi
      * test inline additionalProperties
      *
      * @param  array<string,string> $request_body request body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testInlineAdditionalProperties'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testInlineAdditionalPropertiesWithHttpInfo($request_body)
+    public function testInlineAdditionalPropertiesWithHttpInfo($request_body, string $contentType = self::contentTypes['testInlineAdditionalProperties'][0])
     {
-        $request = $this->testInlineAdditionalPropertiesRequest($request_body);
+        $request = $this->testInlineAdditionalPropertiesRequest($request_body, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -4084,13 +4159,14 @@ class FakeApi
      * test inline additionalProperties
      *
      * @param  array<string,string> $request_body request body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testInlineAdditionalProperties'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testInlineAdditionalPropertiesAsync($request_body)
+    public function testInlineAdditionalPropertiesAsync($request_body, string $contentType = self::contentTypes['testInlineAdditionalProperties'][0])
     {
-        return $this->testInlineAdditionalPropertiesAsyncWithHttpInfo($request_body)
+        return $this->testInlineAdditionalPropertiesAsyncWithHttpInfo($request_body, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -4104,14 +4180,15 @@ class FakeApi
      * test inline additionalProperties
      *
      * @param  array<string,string> $request_body request body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testInlineAdditionalProperties'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testInlineAdditionalPropertiesAsyncWithHttpInfo($request_body)
+    public function testInlineAdditionalPropertiesAsyncWithHttpInfo($request_body, string $contentType = self::contentTypes['testInlineAdditionalProperties'][0])
     {
         $returnType = '';
-        $request = $this->testInlineAdditionalPropertiesRequest($request_body);
+        $request = $this->testInlineAdditionalPropertiesRequest($request_body, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -4140,11 +4217,12 @@ class FakeApi
      * Create request for operation 'testInlineAdditionalProperties'
      *
      * @param  array<string,string> $request_body request body (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testInlineAdditionalProperties'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testInlineAdditionalPropertiesRequest($request_body)
+    public function testInlineAdditionalPropertiesRequest($request_body, string $contentType = self::contentTypes['testInlineAdditionalProperties'][0])
     {
 
         // verify the required parameter 'request_body' is set
@@ -4154,6 +4232,7 @@ class FakeApi
             );
         }
 
+
         $resourcePath = '/fake/inline-additionalProperties';
         $formParams = [];
         $queryParams = [];
@@ -4165,20 +4244,16 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($request_body)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($request_body));
             } else {
                 $httpBody = $request_body;
@@ -4198,9 +4273,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -4236,14 +4311,15 @@ class FakeApi
      *
      * @param  string $param field1 (required)
      * @param  string $param2 field2 (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testJsonFormData'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testJsonFormData($param, $param2)
+    public function testJsonFormData($param, $param2, string $contentType = self::contentTypes['testJsonFormData'][0])
     {
-        $this->testJsonFormDataWithHttpInfo($param, $param2);
+        $this->testJsonFormDataWithHttpInfo($param, $param2, $contentType);
     }
 
     /**
@@ -4253,14 +4329,15 @@ class FakeApi
      *
      * @param  string $param field1 (required)
      * @param  string $param2 field2 (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testJsonFormData'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testJsonFormDataWithHttpInfo($param, $param2)
+    public function testJsonFormDataWithHttpInfo($param, $param2, string $contentType = self::contentTypes['testJsonFormData'][0])
     {
-        $request = $this->testJsonFormDataRequest($param, $param2);
+        $request = $this->testJsonFormDataRequest($param, $param2, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -4313,13 +4390,14 @@ class FakeApi
      *
      * @param  string $param field1 (required)
      * @param  string $param2 field2 (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testJsonFormData'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testJsonFormDataAsync($param, $param2)
+    public function testJsonFormDataAsync($param, $param2, string $contentType = self::contentTypes['testJsonFormData'][0])
     {
-        return $this->testJsonFormDataAsyncWithHttpInfo($param, $param2)
+        return $this->testJsonFormDataAsyncWithHttpInfo($param, $param2, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -4334,14 +4412,15 @@ class FakeApi
      *
      * @param  string $param field1 (required)
      * @param  string $param2 field2 (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testJsonFormData'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testJsonFormDataAsyncWithHttpInfo($param, $param2)
+    public function testJsonFormDataAsyncWithHttpInfo($param, $param2, string $contentType = self::contentTypes['testJsonFormData'][0])
     {
         $returnType = '';
-        $request = $this->testJsonFormDataRequest($param, $param2);
+        $request = $this->testJsonFormDataRequest($param, $param2, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -4371,11 +4450,12 @@ class FakeApi
      *
      * @param  string $param field1 (required)
      * @param  string $param2 field2 (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testJsonFormData'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testJsonFormDataRequest($param, $param2)
+    public function testJsonFormDataRequest($param, $param2, string $contentType = self::contentTypes['testJsonFormData'][0])
     {
 
         // verify the required parameter 'param' is set
@@ -4392,6 +4472,7 @@ class FakeApi
             );
         }
 
+
         $resourcePath = '/fake/jsonFormData';
         $formParams = [];
         $queryParams = [];
@@ -4411,16 +4492,11 @@ class FakeApi
             $formParams['param2'] = ObjectSerializer::toFormValue($param2);
         }
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/x-www-form-urlencoded']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -4438,9 +4514,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -4479,14 +4555,15 @@ class FakeApi
      * @param  string[] $context context (required)
      * @param  string $allow_empty allow_empty (required)
      * @param  array<string,string> $language language (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testQueryParameterCollectionFormat'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function testQueryParameterCollectionFormat($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null)
+    public function testQueryParameterCollectionFormat($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null, string $contentType = self::contentTypes['testQueryParameterCollectionFormat'][0])
     {
-        $this->testQueryParameterCollectionFormatWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language);
+        $this->testQueryParameterCollectionFormatWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language, $contentType);
     }
 
     /**
@@ -4499,14 +4576,15 @@ class FakeApi
      * @param  string[] $context (required)
      * @param  string $allow_empty (required)
      * @param  array<string,string> $language (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testQueryParameterCollectionFormat'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testQueryParameterCollectionFormatWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null)
+    public function testQueryParameterCollectionFormatWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null, string $contentType = self::contentTypes['testQueryParameterCollectionFormat'][0])
     {
-        $request = $this->testQueryParameterCollectionFormatRequest($pipe, $ioutil, $http, $url, $context, $allow_empty, $language);
+        $request = $this->testQueryParameterCollectionFormatRequest($pipe, $ioutil, $http, $url, $context, $allow_empty, $language, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -4562,13 +4640,14 @@ class FakeApi
      * @param  string[] $context (required)
      * @param  string $allow_empty (required)
      * @param  array<string,string> $language (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testQueryParameterCollectionFormat'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testQueryParameterCollectionFormatAsync($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null)
+    public function testQueryParameterCollectionFormatAsync($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null, string $contentType = self::contentTypes['testQueryParameterCollectionFormat'][0])
     {
-        return $this->testQueryParameterCollectionFormatAsyncWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language)
+        return $this->testQueryParameterCollectionFormatAsyncWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -4586,14 +4665,15 @@ class FakeApi
      * @param  string[] $context (required)
      * @param  string $allow_empty (required)
      * @param  array<string,string> $language (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testQueryParameterCollectionFormat'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testQueryParameterCollectionFormatAsyncWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null)
+    public function testQueryParameterCollectionFormatAsyncWithHttpInfo($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null, string $contentType = self::contentTypes['testQueryParameterCollectionFormat'][0])
     {
         $returnType = '';
-        $request = $this->testQueryParameterCollectionFormatRequest($pipe, $ioutil, $http, $url, $context, $allow_empty, $language);
+        $request = $this->testQueryParameterCollectionFormatRequest($pipe, $ioutil, $http, $url, $context, $allow_empty, $language, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -4628,11 +4708,12 @@ class FakeApi
      * @param  string[] $context (required)
      * @param  string $allow_empty (required)
      * @param  array<string,string> $language (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testQueryParameterCollectionFormat'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testQueryParameterCollectionFormatRequest($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null)
+    public function testQueryParameterCollectionFormatRequest($pipe, $ioutil, $http, $url, $context, $allow_empty, $language = null, string $contentType = self::contentTypes['testQueryParameterCollectionFormat'][0])
     {
 
         // verify the required parameter 'pipe' is set
@@ -4678,6 +4759,7 @@ class FakeApi
         }
 
 
+
         $resourcePath = '/fake/test-query-parameters';
         $formParams = [];
         $queryParams = [];
@@ -4752,16 +4834,11 @@ class FakeApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -4779,9 +4856,9 @@ class FakeApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
index 0dcd17977ad..47e1247402a 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
@@ -69,7 +69,14 @@ class FakeClassnameTags123Api
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [
+        'testClassname' => [
+            'application/json',
+        ],
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -121,14 +128,15 @@ class FakeClassnameTags123Api
      * To test class name in snake case
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClassname'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Client
      */
-    public function testClassname($client)
+    public function testClassname($client, string $contentType = self::contentTypes['testClassname'][0])
     {
-        list($response) = $this->testClassnameWithHttpInfo($client);
+        list($response) = $this->testClassnameWithHttpInfo($client, $contentType);
         return $response;
     }
 
@@ -138,14 +146,15 @@ class FakeClassnameTags123Api
      * To test class name in snake case
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClassname'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Client, HTTP status code, HTTP response headers (array of strings)
      */
-    public function testClassnameWithHttpInfo($client)
+    public function testClassnameWithHttpInfo($client, string $contentType = self::contentTypes['testClassname'][0])
     {
-        $request = $this->testClassnameRequest($client);
+        $request = $this->testClassnameRequest($client, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -237,13 +246,14 @@ class FakeClassnameTags123Api
      * To test class name in snake case
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClassname'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testClassnameAsync($client)
+    public function testClassnameAsync($client, string $contentType = self::contentTypes['testClassname'][0])
     {
-        return $this->testClassnameAsyncWithHttpInfo($client)
+        return $this->testClassnameAsyncWithHttpInfo($client, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -257,14 +267,15 @@ class FakeClassnameTags123Api
      * To test class name in snake case
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClassname'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function testClassnameAsyncWithHttpInfo($client)
+    public function testClassnameAsyncWithHttpInfo($client, string $contentType = self::contentTypes['testClassname'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Client';
-        $request = $this->testClassnameRequest($client);
+        $request = $this->testClassnameRequest($client, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -306,11 +317,12 @@ class FakeClassnameTags123Api
      * Create request for operation 'testClassname'
      *
      * @param  \OpenAPI\Client\Model\Client $client client model (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['testClassname'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function testClassnameRequest($client)
+    public function testClassnameRequest($client, string $contentType = self::contentTypes['testClassname'][0])
     {
 
         // verify the required parameter 'client' is set
@@ -320,6 +332,7 @@ class FakeClassnameTags123Api
             );
         }
 
+
         $resourcePath = '/fake_classname_test';
         $formParams = [];
         $queryParams = [];
@@ -331,20 +344,16 @@ class FakeClassnameTags123Api
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($client)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($client));
             } else {
                 $httpBody = $client;
@@ -364,9 +373,9 @@ class FakeClassnameTags123Api
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
index bc00b5ed681..b818c0a819a 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
@@ -69,7 +69,40 @@ class PetApi
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [
+        'addPet' => [
+            'application/json',
+            'application/xml',
+        ],
+        'deletePet' => [
+            'application/json',
+        ],
+        'findPetsByStatus' => [
+            'application/json',
+        ],
+        'findPetsByTags' => [
+            'application/json',
+        ],
+        'getPetById' => [
+            'application/json',
+        ],
+        'updatePet' => [
+            'application/json',
+            'application/xml',
+        ],
+        'updatePetWithForm' => [
+            'application/x-www-form-urlencoded',
+        ],
+        'uploadFile' => [
+            'multipart/form-data',
+        ],
+        'uploadFileWithRequiredFile' => [
+            'multipart/form-data',
+        ],
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -139,14 +172,15 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['addPet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function addPet($pet, ?int $hostIndex = null, array $variables = [])
+    public function addPet($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['addPet'][0])
     {
-        $this->addPetWithHttpInfo($pet, $hostIndex, $variables);
+        $this->addPetWithHttpInfo($pet, $hostIndex, $variables, $contentType);
     }
 
     /**
@@ -173,14 +207,15 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['addPet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function addPetWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [])
+    public function addPetWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['addPet'][0])
     {
-        $request = $this->addPetRequest($pet, $hostIndex, $variables);
+        $request = $this->addPetRequest($pet, $hostIndex, $variables, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -250,13 +285,14 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['addPet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function addPetAsync($pet, ?int $hostIndex = null, array $variables = [])
+    public function addPetAsync($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['addPet'][0])
     {
-        return $this->addPetAsyncWithHttpInfo($pet, $hostIndex, $variables)
+        return $this->addPetAsyncWithHttpInfo($pet, $hostIndex, $variables, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -288,14 +324,15 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['addPet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function addPetAsyncWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [])
+    public function addPetAsyncWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['addPet'][0])
     {
         $returnType = '';
-        $request = $this->addPetRequest($pet, $hostIndex, $variables);
+        $request = $this->addPetRequest($pet, $hostIndex, $variables, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -342,11 +379,12 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['addPet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function addPetRequest($pet, ?int $hostIndex = null, array $variables = [])
+    public function addPetRequest($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['addPet'][0])
     {
 
         // verify the required parameter 'pet' is set
@@ -356,6 +394,7 @@ class PetApi
             );
         }
 
+
         $resourcePath = '/pet';
         $formParams = [];
         $queryParams = [];
@@ -367,20 +406,16 @@ class PetApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json', 'application/xml']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($pet)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($pet));
             } else {
                 $httpBody = $pet;
@@ -400,9 +435,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -494,14 +529,15 @@ class PetApi
      *
      * @param  int $pet_id Pet id to delete (required)
      * @param  string $api_key api_key (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deletePet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function deletePet($pet_id, $api_key = null)
+    public function deletePet($pet_id, $api_key = null, string $contentType = self::contentTypes['deletePet'][0])
     {
-        $this->deletePetWithHttpInfo($pet_id, $api_key);
+        $this->deletePetWithHttpInfo($pet_id, $api_key, $contentType);
     }
 
     /**
@@ -511,14 +547,15 @@ class PetApi
      *
      * @param  int $pet_id Pet id to delete (required)
      * @param  string $api_key (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deletePet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function deletePetWithHttpInfo($pet_id, $api_key = null)
+    public function deletePetWithHttpInfo($pet_id, $api_key = null, string $contentType = self::contentTypes['deletePet'][0])
     {
-        $request = $this->deletePetRequest($pet_id, $api_key);
+        $request = $this->deletePetRequest($pet_id, $api_key, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -571,13 +608,14 @@ class PetApi
      *
      * @param  int $pet_id Pet id to delete (required)
      * @param  string $api_key (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deletePet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function deletePetAsync($pet_id, $api_key = null)
+    public function deletePetAsync($pet_id, $api_key = null, string $contentType = self::contentTypes['deletePet'][0])
     {
-        return $this->deletePetAsyncWithHttpInfo($pet_id, $api_key)
+        return $this->deletePetAsyncWithHttpInfo($pet_id, $api_key, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -592,14 +630,15 @@ class PetApi
      *
      * @param  int $pet_id Pet id to delete (required)
      * @param  string $api_key (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deletePet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function deletePetAsyncWithHttpInfo($pet_id, $api_key = null)
+    public function deletePetAsyncWithHttpInfo($pet_id, $api_key = null, string $contentType = self::contentTypes['deletePet'][0])
     {
         $returnType = '';
-        $request = $this->deletePetRequest($pet_id, $api_key);
+        $request = $this->deletePetRequest($pet_id, $api_key, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -629,11 +668,12 @@ class PetApi
      *
      * @param  int $pet_id Pet id to delete (required)
      * @param  string $api_key (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deletePet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function deletePetRequest($pet_id, $api_key = null)
+    public function deletePetRequest($pet_id, $api_key = null, string $contentType = self::contentTypes['deletePet'][0])
     {
 
         // verify the required parameter 'pet_id' is set
@@ -644,6 +684,7 @@ class PetApi
         }
 
 
+
         $resourcePath = '/pet/{petId}';
         $formParams = [];
         $queryParams = [];
@@ -667,16 +708,11 @@ class PetApi
         }
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -694,9 +730,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -735,14 +771,15 @@ class PetApi
      * Finds Pets by status
      *
      * @param  string[] $status Status values that need to be considered for filter (required) (deprecated)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByStatus'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Pet[]
      */
-    public function findPetsByStatus($status)
+    public function findPetsByStatus($status, string $contentType = self::contentTypes['findPetsByStatus'][0])
     {
-        list($response) = $this->findPetsByStatusWithHttpInfo($status);
+        list($response) = $this->findPetsByStatusWithHttpInfo($status, $contentType);
         return $response;
     }
 
@@ -752,14 +789,15 @@ class PetApi
      * Finds Pets by status
      *
      * @param  string[] $status Status values that need to be considered for filter (required) (deprecated)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByStatus'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Pet[], HTTP status code, HTTP response headers (array of strings)
      */
-    public function findPetsByStatusWithHttpInfo($status)
+    public function findPetsByStatusWithHttpInfo($status, string $contentType = self::contentTypes['findPetsByStatus'][0])
     {
-        $request = $this->findPetsByStatusRequest($status);
+        $request = $this->findPetsByStatusRequest($status, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -851,13 +889,14 @@ class PetApi
      * Finds Pets by status
      *
      * @param  string[] $status Status values that need to be considered for filter (required) (deprecated)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByStatus'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function findPetsByStatusAsync($status)
+    public function findPetsByStatusAsync($status, string $contentType = self::contentTypes['findPetsByStatus'][0])
     {
-        return $this->findPetsByStatusAsyncWithHttpInfo($status)
+        return $this->findPetsByStatusAsyncWithHttpInfo($status, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -871,14 +910,15 @@ class PetApi
      * Finds Pets by status
      *
      * @param  string[] $status Status values that need to be considered for filter (required) (deprecated)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByStatus'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function findPetsByStatusAsyncWithHttpInfo($status)
+    public function findPetsByStatusAsyncWithHttpInfo($status, string $contentType = self::contentTypes['findPetsByStatus'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Pet[]';
-        $request = $this->findPetsByStatusRequest($status);
+        $request = $this->findPetsByStatusRequest($status, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -920,11 +960,12 @@ class PetApi
      * Create request for operation 'findPetsByStatus'
      *
      * @param  string[] $status Status values that need to be considered for filter (required) (deprecated)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByStatus'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function findPetsByStatusRequest($status)
+    public function findPetsByStatusRequest($status, string $contentType = self::contentTypes['findPetsByStatus'][0])
     {
 
         // verify the required parameter 'status' is set
@@ -934,6 +975,7 @@ class PetApi
             );
         }
 
+
         $resourcePath = '/pet/findByStatus';
         $formParams = [];
         $queryParams = [];
@@ -954,16 +996,11 @@ class PetApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/xml', 'application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/xml', 'application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/xml', 'application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -981,9 +1018,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1022,15 +1059,16 @@ class PetApi
      * Finds Pets by tags
      *
      * @param  string[] $tags Tags to filter by (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByTags'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Pet[]
      * @deprecated
      */
-    public function findPetsByTags($tags)
+    public function findPetsByTags($tags, string $contentType = self::contentTypes['findPetsByTags'][0])
     {
-        list($response) = $this->findPetsByTagsWithHttpInfo($tags);
+        list($response) = $this->findPetsByTagsWithHttpInfo($tags, $contentType);
         return $response;
     }
 
@@ -1040,15 +1078,16 @@ class PetApi
      * Finds Pets by tags
      *
      * @param  string[] $tags Tags to filter by (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByTags'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Pet[], HTTP status code, HTTP response headers (array of strings)
      * @deprecated
      */
-    public function findPetsByTagsWithHttpInfo($tags)
+    public function findPetsByTagsWithHttpInfo($tags, string $contentType = self::contentTypes['findPetsByTags'][0])
     {
-        $request = $this->findPetsByTagsRequest($tags);
+        $request = $this->findPetsByTagsRequest($tags, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1140,14 +1179,15 @@ class PetApi
      * Finds Pets by tags
      *
      * @param  string[] $tags Tags to filter by (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByTags'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      * @deprecated
      */
-    public function findPetsByTagsAsync($tags)
+    public function findPetsByTagsAsync($tags, string $contentType = self::contentTypes['findPetsByTags'][0])
     {
-        return $this->findPetsByTagsAsyncWithHttpInfo($tags)
+        return $this->findPetsByTagsAsyncWithHttpInfo($tags, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1161,15 +1201,16 @@ class PetApi
      * Finds Pets by tags
      *
      * @param  string[] $tags Tags to filter by (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByTags'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      * @deprecated
      */
-    public function findPetsByTagsAsyncWithHttpInfo($tags)
+    public function findPetsByTagsAsyncWithHttpInfo($tags, string $contentType = self::contentTypes['findPetsByTags'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Pet[]';
-        $request = $this->findPetsByTagsRequest($tags);
+        $request = $this->findPetsByTagsRequest($tags, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1211,12 +1252,13 @@ class PetApi
      * Create request for operation 'findPetsByTags'
      *
      * @param  string[] $tags Tags to filter by (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['findPetsByTags'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      * @deprecated
      */
-    public function findPetsByTagsRequest($tags)
+    public function findPetsByTagsRequest($tags, string $contentType = self::contentTypes['findPetsByTags'][0])
     {
 
         // verify the required parameter 'tags' is set
@@ -1225,7 +1267,7 @@ class PetApi
                 'Missing the required parameter $tags when calling findPetsByTags'
             );
         }
-
+        
 
         $resourcePath = '/pet/findByTags';
         $formParams = [];
@@ -1247,16 +1289,11 @@ class PetApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/xml', 'application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/xml', 'application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/xml', 'application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -1274,9 +1311,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1315,14 +1352,15 @@ class PetApi
      * Find pet by ID
      *
      * @param  int $pet_id ID of pet to return (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getPetById'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Pet
      */
-    public function getPetById($pet_id)
+    public function getPetById($pet_id, string $contentType = self::contentTypes['getPetById'][0])
     {
-        list($response) = $this->getPetByIdWithHttpInfo($pet_id);
+        list($response) = $this->getPetByIdWithHttpInfo($pet_id, $contentType);
         return $response;
     }
 
@@ -1332,14 +1370,15 @@ class PetApi
      * Find pet by ID
      *
      * @param  int $pet_id ID of pet to return (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getPetById'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Pet, HTTP status code, HTTP response headers (array of strings)
      */
-    public function getPetByIdWithHttpInfo($pet_id)
+    public function getPetByIdWithHttpInfo($pet_id, string $contentType = self::contentTypes['getPetById'][0])
     {
-        $request = $this->getPetByIdRequest($pet_id);
+        $request = $this->getPetByIdRequest($pet_id, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1431,13 +1470,14 @@ class PetApi
      * Find pet by ID
      *
      * @param  int $pet_id ID of pet to return (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getPetById'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getPetByIdAsync($pet_id)
+    public function getPetByIdAsync($pet_id, string $contentType = self::contentTypes['getPetById'][0])
     {
-        return $this->getPetByIdAsyncWithHttpInfo($pet_id)
+        return $this->getPetByIdAsyncWithHttpInfo($pet_id, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1451,14 +1491,15 @@ class PetApi
      * Find pet by ID
      *
      * @param  int $pet_id ID of pet to return (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getPetById'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getPetByIdAsyncWithHttpInfo($pet_id)
+    public function getPetByIdAsyncWithHttpInfo($pet_id, string $contentType = self::contentTypes['getPetById'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Pet';
-        $request = $this->getPetByIdRequest($pet_id);
+        $request = $this->getPetByIdRequest($pet_id, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1500,11 +1541,12 @@ class PetApi
      * Create request for operation 'getPetById'
      *
      * @param  int $pet_id ID of pet to return (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getPetById'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function getPetByIdRequest($pet_id)
+    public function getPetByIdRequest($pet_id, string $contentType = self::contentTypes['getPetById'][0])
     {
 
         // verify the required parameter 'pet_id' is set
@@ -1514,6 +1556,7 @@ class PetApi
             );
         }
 
+
         $resourcePath = '/pet/{petId}';
         $formParams = [];
         $queryParams = [];
@@ -1533,16 +1576,11 @@ class PetApi
         }
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/xml', 'application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/xml', 'application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/xml', 'application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -1560,9 +1598,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1620,14 +1658,15 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function updatePet($pet, ?int $hostIndex = null, array $variables = [])
+    public function updatePet($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['updatePet'][0])
     {
-        $this->updatePetWithHttpInfo($pet, $hostIndex, $variables);
+        $this->updatePetWithHttpInfo($pet, $hostIndex, $variables, $contentType);
     }
 
     /**
@@ -1654,14 +1693,15 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePet'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function updatePetWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [])
+    public function updatePetWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['updatePet'][0])
     {
-        $request = $this->updatePetRequest($pet, $hostIndex, $variables);
+        $request = $this->updatePetRequest($pet, $hostIndex, $variables, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1731,13 +1771,14 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function updatePetAsync($pet, ?int $hostIndex = null, array $variables = [])
+    public function updatePetAsync($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['updatePet'][0])
     {
-        return $this->updatePetAsyncWithHttpInfo($pet, $hostIndex, $variables)
+        return $this->updatePetAsyncWithHttpInfo($pet, $hostIndex, $variables, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1769,14 +1810,15 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function updatePetAsyncWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [])
+    public function updatePetAsyncWithHttpInfo($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['updatePet'][0])
     {
         $returnType = '';
-        $request = $this->updatePetRequest($pet, $hostIndex, $variables);
+        $request = $this->updatePetRequest($pet, $hostIndex, $variables, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1823,11 +1865,12 @@ class PetApi
      * @param  \OpenAPI\Client\Model\Pet $pet Pet object that needs to be added to the store (required)
      * @param  null|int $hostIndex Host index. Defaults to null. If null, then the library will use $this->hostIndex instead
      * @param  array $variables Associative array of variables to pass to the host. Defaults to empty array.
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePet'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function updatePetRequest($pet, ?int $hostIndex = null, array $variables = [])
+    public function updatePetRequest($pet, ?int $hostIndex = null, array $variables = [], string $contentType = self::contentTypes['updatePet'][0])
     {
 
         // verify the required parameter 'pet' is set
@@ -1837,6 +1880,7 @@ class PetApi
             );
         }
 
+
         $resourcePath = '/pet';
         $formParams = [];
         $queryParams = [];
@@ -1848,20 +1892,16 @@ class PetApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json', 'application/xml']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($pet)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($pet));
             } else {
                 $httpBody = $pet;
@@ -1881,9 +1921,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1976,14 +2016,15 @@ class PetApi
      * @param  int $pet_id ID of pet that needs to be updated (required)
      * @param  string $name Updated name of the pet (optional)
      * @param  string $status Updated status of the pet (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePetWithForm'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function updatePetWithForm($pet_id, $name = null, $status = null)
+    public function updatePetWithForm($pet_id, $name = null, $status = null, string $contentType = self::contentTypes['updatePetWithForm'][0])
     {
-        $this->updatePetWithFormWithHttpInfo($pet_id, $name, $status);
+        $this->updatePetWithFormWithHttpInfo($pet_id, $name, $status, $contentType);
     }
 
     /**
@@ -1994,14 +2035,15 @@ class PetApi
      * @param  int $pet_id ID of pet that needs to be updated (required)
      * @param  string $name Updated name of the pet (optional)
      * @param  string $status Updated status of the pet (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePetWithForm'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function updatePetWithFormWithHttpInfo($pet_id, $name = null, $status = null)
+    public function updatePetWithFormWithHttpInfo($pet_id, $name = null, $status = null, string $contentType = self::contentTypes['updatePetWithForm'][0])
     {
-        $request = $this->updatePetWithFormRequest($pet_id, $name, $status);
+        $request = $this->updatePetWithFormRequest($pet_id, $name, $status, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -2055,13 +2097,14 @@ class PetApi
      * @param  int $pet_id ID of pet that needs to be updated (required)
      * @param  string $name Updated name of the pet (optional)
      * @param  string $status Updated status of the pet (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePetWithForm'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function updatePetWithFormAsync($pet_id, $name = null, $status = null)
+    public function updatePetWithFormAsync($pet_id, $name = null, $status = null, string $contentType = self::contentTypes['updatePetWithForm'][0])
     {
-        return $this->updatePetWithFormAsyncWithHttpInfo($pet_id, $name, $status)
+        return $this->updatePetWithFormAsyncWithHttpInfo($pet_id, $name, $status, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -2077,14 +2120,15 @@ class PetApi
      * @param  int $pet_id ID of pet that needs to be updated (required)
      * @param  string $name Updated name of the pet (optional)
      * @param  string $status Updated status of the pet (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePetWithForm'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function updatePetWithFormAsyncWithHttpInfo($pet_id, $name = null, $status = null)
+    public function updatePetWithFormAsyncWithHttpInfo($pet_id, $name = null, $status = null, string $contentType = self::contentTypes['updatePetWithForm'][0])
     {
         $returnType = '';
-        $request = $this->updatePetWithFormRequest($pet_id, $name, $status);
+        $request = $this->updatePetWithFormRequest($pet_id, $name, $status, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -2115,11 +2159,12 @@ class PetApi
      * @param  int $pet_id ID of pet that needs to be updated (required)
      * @param  string $name Updated name of the pet (optional)
      * @param  string $status Updated status of the pet (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updatePetWithForm'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function updatePetWithFormRequest($pet_id, $name = null, $status = null)
+    public function updatePetWithFormRequest($pet_id, $name = null, $status = null, string $contentType = self::contentTypes['updatePetWithForm'][0])
     {
 
         // verify the required parameter 'pet_id' is set
@@ -2131,6 +2176,7 @@ class PetApi
 
 
 
+
         $resourcePath = '/pet/{petId}';
         $formParams = [];
         $queryParams = [];
@@ -2158,16 +2204,11 @@ class PetApi
             $formParams['status'] = ObjectSerializer::toFormValue($status);
         }
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/x-www-form-urlencoded']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -2185,9 +2226,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -2228,14 +2269,15 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
      * @param  \SplFileObject $file file to upload (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFile'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\ApiResponse
      */
-    public function uploadFile($pet_id, $additional_metadata = null, $file = null)
+    public function uploadFile($pet_id, $additional_metadata = null, $file = null, string $contentType = self::contentTypes['uploadFile'][0])
     {
-        list($response) = $this->uploadFileWithHttpInfo($pet_id, $additional_metadata, $file);
+        list($response) = $this->uploadFileWithHttpInfo($pet_id, $additional_metadata, $file, $contentType);
         return $response;
     }
 
@@ -2247,14 +2289,15 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
      * @param  \SplFileObject $file file to upload (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFile'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\ApiResponse, HTTP status code, HTTP response headers (array of strings)
      */
-    public function uploadFileWithHttpInfo($pet_id, $additional_metadata = null, $file = null)
+    public function uploadFileWithHttpInfo($pet_id, $additional_metadata = null, $file = null, string $contentType = self::contentTypes['uploadFile'][0])
     {
-        $request = $this->uploadFileRequest($pet_id, $additional_metadata, $file);
+        $request = $this->uploadFileRequest($pet_id, $additional_metadata, $file, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -2348,13 +2391,14 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
      * @param  \SplFileObject $file file to upload (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFile'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function uploadFileAsync($pet_id, $additional_metadata = null, $file = null)
+    public function uploadFileAsync($pet_id, $additional_metadata = null, $file = null, string $contentType = self::contentTypes['uploadFile'][0])
     {
-        return $this->uploadFileAsyncWithHttpInfo($pet_id, $additional_metadata, $file)
+        return $this->uploadFileAsyncWithHttpInfo($pet_id, $additional_metadata, $file, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -2370,14 +2414,15 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
      * @param  \SplFileObject $file file to upload (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFile'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function uploadFileAsyncWithHttpInfo($pet_id, $additional_metadata = null, $file = null)
+    public function uploadFileAsyncWithHttpInfo($pet_id, $additional_metadata = null, $file = null, string $contentType = self::contentTypes['uploadFile'][0])
     {
         $returnType = '\OpenAPI\Client\Model\ApiResponse';
-        $request = $this->uploadFileRequest($pet_id, $additional_metadata, $file);
+        $request = $this->uploadFileRequest($pet_id, $additional_metadata, $file, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -2421,11 +2466,12 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
      * @param  \SplFileObject $file file to upload (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFile'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function uploadFileRequest($pet_id, $additional_metadata = null, $file = null)
+    public function uploadFileRequest($pet_id, $additional_metadata = null, $file = null, string $contentType = self::contentTypes['uploadFile'][0])
     {
 
         // verify the required parameter 'pet_id' is set
@@ -2437,6 +2483,7 @@ class PetApi
 
 
 
+
         $resourcePath = '/pet/{petId}/uploadImage';
         $formParams = [];
         $queryParams = [];
@@ -2472,16 +2519,11 @@ class PetApi
             }
         }
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                ['multipart/form-data']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -2499,9 +2541,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -2542,14 +2584,15 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  \SplFileObject $required_file file to upload (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFileWithRequiredFile'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\ApiResponse
      */
-    public function uploadFileWithRequiredFile($pet_id, $required_file, $additional_metadata = null)
+    public function uploadFileWithRequiredFile($pet_id, $required_file, $additional_metadata = null, string $contentType = self::contentTypes['uploadFileWithRequiredFile'][0])
     {
-        list($response) = $this->uploadFileWithRequiredFileWithHttpInfo($pet_id, $required_file, $additional_metadata);
+        list($response) = $this->uploadFileWithRequiredFileWithHttpInfo($pet_id, $required_file, $additional_metadata, $contentType);
         return $response;
     }
 
@@ -2561,14 +2604,15 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  \SplFileObject $required_file file to upload (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFileWithRequiredFile'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\ApiResponse, HTTP status code, HTTP response headers (array of strings)
      */
-    public function uploadFileWithRequiredFileWithHttpInfo($pet_id, $required_file, $additional_metadata = null)
+    public function uploadFileWithRequiredFileWithHttpInfo($pet_id, $required_file, $additional_metadata = null, string $contentType = self::contentTypes['uploadFileWithRequiredFile'][0])
     {
-        $request = $this->uploadFileWithRequiredFileRequest($pet_id, $required_file, $additional_metadata);
+        $request = $this->uploadFileWithRequiredFileRequest($pet_id, $required_file, $additional_metadata, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -2662,13 +2706,14 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  \SplFileObject $required_file file to upload (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFileWithRequiredFile'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function uploadFileWithRequiredFileAsync($pet_id, $required_file, $additional_metadata = null)
+    public function uploadFileWithRequiredFileAsync($pet_id, $required_file, $additional_metadata = null, string $contentType = self::contentTypes['uploadFileWithRequiredFile'][0])
     {
-        return $this->uploadFileWithRequiredFileAsyncWithHttpInfo($pet_id, $required_file, $additional_metadata)
+        return $this->uploadFileWithRequiredFileAsyncWithHttpInfo($pet_id, $required_file, $additional_metadata, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -2684,14 +2729,15 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  \SplFileObject $required_file file to upload (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFileWithRequiredFile'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function uploadFileWithRequiredFileAsyncWithHttpInfo($pet_id, $required_file, $additional_metadata = null)
+    public function uploadFileWithRequiredFileAsyncWithHttpInfo($pet_id, $required_file, $additional_metadata = null, string $contentType = self::contentTypes['uploadFileWithRequiredFile'][0])
     {
         $returnType = '\OpenAPI\Client\Model\ApiResponse';
-        $request = $this->uploadFileWithRequiredFileRequest($pet_id, $required_file, $additional_metadata);
+        $request = $this->uploadFileWithRequiredFileRequest($pet_id, $required_file, $additional_metadata, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -2735,11 +2781,12 @@ class PetApi
      * @param  int $pet_id ID of pet to update (required)
      * @param  \SplFileObject $required_file file to upload (required)
      * @param  string $additional_metadata Additional data to pass to server (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadFileWithRequiredFile'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function uploadFileWithRequiredFileRequest($pet_id, $required_file, $additional_metadata = null)
+    public function uploadFileWithRequiredFileRequest($pet_id, $required_file, $additional_metadata = null, string $contentType = self::contentTypes['uploadFileWithRequiredFile'][0])
     {
 
         // verify the required parameter 'pet_id' is set
@@ -2757,6 +2804,7 @@ class PetApi
         }
 
 
+
         $resourcePath = '/fake/{petId}/uploadImageWithRequiredFile';
         $formParams = [];
         $queryParams = [];
@@ -2792,16 +2840,11 @@ class PetApi
             }
         }
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                ['multipart/form-data']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -2819,9 +2862,9 @@ class PetApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
index b4e44d05a30..f238db0c1bb 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
@@ -69,7 +69,23 @@ class StoreApi
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [
+        'deleteOrder' => [
+            'application/json',
+        ],
+        'getInventory' => [
+            'application/json',
+        ],
+        'getOrderById' => [
+            'application/json',
+        ],
+        'placeOrder' => [
+            'application/json',
+        ],
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -121,14 +137,15 @@ class StoreApi
      * Delete purchase order by ID
      *
      * @param  string $order_id ID of the order that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteOrder'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function deleteOrder($order_id)
+    public function deleteOrder($order_id, string $contentType = self::contentTypes['deleteOrder'][0])
     {
-        $this->deleteOrderWithHttpInfo($order_id);
+        $this->deleteOrderWithHttpInfo($order_id, $contentType);
     }
 
     /**
@@ -137,14 +154,15 @@ class StoreApi
      * Delete purchase order by ID
      *
      * @param  string $order_id ID of the order that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteOrder'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function deleteOrderWithHttpInfo($order_id)
+    public function deleteOrderWithHttpInfo($order_id, string $contentType = self::contentTypes['deleteOrder'][0])
     {
-        $request = $this->deleteOrderRequest($order_id);
+        $request = $this->deleteOrderRequest($order_id, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -196,13 +214,14 @@ class StoreApi
      * Delete purchase order by ID
      *
      * @param  string $order_id ID of the order that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteOrder'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function deleteOrderAsync($order_id)
+    public function deleteOrderAsync($order_id, string $contentType = self::contentTypes['deleteOrder'][0])
     {
-        return $this->deleteOrderAsyncWithHttpInfo($order_id)
+        return $this->deleteOrderAsyncWithHttpInfo($order_id, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -216,14 +235,15 @@ class StoreApi
      * Delete purchase order by ID
      *
      * @param  string $order_id ID of the order that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteOrder'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function deleteOrderAsyncWithHttpInfo($order_id)
+    public function deleteOrderAsyncWithHttpInfo($order_id, string $contentType = self::contentTypes['deleteOrder'][0])
     {
         $returnType = '';
-        $request = $this->deleteOrderRequest($order_id);
+        $request = $this->deleteOrderRequest($order_id, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -252,11 +272,12 @@ class StoreApi
      * Create request for operation 'deleteOrder'
      *
      * @param  string $order_id ID of the order that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteOrder'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function deleteOrderRequest($order_id)
+    public function deleteOrderRequest($order_id, string $contentType = self::contentTypes['deleteOrder'][0])
     {
 
         // verify the required parameter 'order_id' is set
@@ -266,6 +287,7 @@ class StoreApi
             );
         }
 
+
         $resourcePath = '/store/order/{order_id}';
         $formParams = [];
         $queryParams = [];
@@ -285,16 +307,11 @@ class StoreApi
         }
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -312,9 +329,9 @@ class StoreApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -348,14 +365,15 @@ class StoreApi
      *
      * Returns pet inventories by status
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getInventory'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array<string,int>
      */
-    public function getInventory()
+    public function getInventory(string $contentType = self::contentTypes['getInventory'][0])
     {
-        list($response) = $this->getInventoryWithHttpInfo();
+        list($response) = $this->getInventoryWithHttpInfo($contentType);
         return $response;
     }
 
@@ -364,14 +382,15 @@ class StoreApi
      *
      * Returns pet inventories by status
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getInventory'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of array<string,int>, HTTP status code, HTTP response headers (array of strings)
      */
-    public function getInventoryWithHttpInfo()
+    public function getInventoryWithHttpInfo(string $contentType = self::contentTypes['getInventory'][0])
     {
-        $request = $this->getInventoryRequest();
+        $request = $this->getInventoryRequest($contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -462,13 +481,14 @@ class StoreApi
      *
      * Returns pet inventories by status
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getInventory'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getInventoryAsync()
+    public function getInventoryAsync(string $contentType = self::contentTypes['getInventory'][0])
     {
-        return $this->getInventoryAsyncWithHttpInfo()
+        return $this->getInventoryAsyncWithHttpInfo($contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -481,14 +501,15 @@ class StoreApi
      *
      * Returns pet inventories by status
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getInventory'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getInventoryAsyncWithHttpInfo()
+    public function getInventoryAsyncWithHttpInfo(string $contentType = self::contentTypes['getInventory'][0])
     {
         $returnType = 'array<string,int>';
-        $request = $this->getInventoryRequest();
+        $request = $this->getInventoryRequest($contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -529,13 +550,15 @@ class StoreApi
     /**
      * Create request for operation 'getInventory'
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getInventory'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function getInventoryRequest()
+    public function getInventoryRequest(string $contentType = self::contentTypes['getInventory'][0])
     {
 
+
         $resourcePath = '/store/inventory';
         $formParams = [];
         $queryParams = [];
@@ -547,16 +570,11 @@ class StoreApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -574,9 +592,9 @@ class StoreApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -616,14 +634,15 @@ class StoreApi
      * Find purchase order by ID
      *
      * @param  int $order_id ID of pet that needs to be fetched (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getOrderById'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Order
      */
-    public function getOrderById($order_id)
+    public function getOrderById($order_id, string $contentType = self::contentTypes['getOrderById'][0])
     {
-        list($response) = $this->getOrderByIdWithHttpInfo($order_id);
+        list($response) = $this->getOrderByIdWithHttpInfo($order_id, $contentType);
         return $response;
     }
 
@@ -633,14 +652,15 @@ class StoreApi
      * Find purchase order by ID
      *
      * @param  int $order_id ID of pet that needs to be fetched (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getOrderById'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Order, HTTP status code, HTTP response headers (array of strings)
      */
-    public function getOrderByIdWithHttpInfo($order_id)
+    public function getOrderByIdWithHttpInfo($order_id, string $contentType = self::contentTypes['getOrderById'][0])
     {
-        $request = $this->getOrderByIdRequest($order_id);
+        $request = $this->getOrderByIdRequest($order_id, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -732,13 +752,14 @@ class StoreApi
      * Find purchase order by ID
      *
      * @param  int $order_id ID of pet that needs to be fetched (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getOrderById'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getOrderByIdAsync($order_id)
+    public function getOrderByIdAsync($order_id, string $contentType = self::contentTypes['getOrderById'][0])
     {
-        return $this->getOrderByIdAsyncWithHttpInfo($order_id)
+        return $this->getOrderByIdAsyncWithHttpInfo($order_id, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -752,14 +773,15 @@ class StoreApi
      * Find purchase order by ID
      *
      * @param  int $order_id ID of pet that needs to be fetched (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getOrderById'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getOrderByIdAsyncWithHttpInfo($order_id)
+    public function getOrderByIdAsyncWithHttpInfo($order_id, string $contentType = self::contentTypes['getOrderById'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Order';
-        $request = $this->getOrderByIdRequest($order_id);
+        $request = $this->getOrderByIdRequest($order_id, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -801,11 +823,12 @@ class StoreApi
      * Create request for operation 'getOrderById'
      *
      * @param  int $order_id ID of pet that needs to be fetched (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getOrderById'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function getOrderByIdRequest($order_id)
+    public function getOrderByIdRequest($order_id, string $contentType = self::contentTypes['getOrderById'][0])
     {
 
         // verify the required parameter 'order_id' is set
@@ -820,7 +843,7 @@ class StoreApi
         if ($order_id < 1) {
             throw new \InvalidArgumentException('invalid value for "$order_id" when calling StoreApi.getOrderById, must be bigger than or equal to 1.');
         }
-
+        
 
         $resourcePath = '/store/order/{order_id}';
         $formParams = [];
@@ -841,16 +864,11 @@ class StoreApi
         }
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/xml', 'application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/xml', 'application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/xml', 'application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -868,9 +886,9 @@ class StoreApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -905,14 +923,15 @@ class StoreApi
      * Place an order for a pet
      *
      * @param  \OpenAPI\Client\Model\Order $order order placed for purchasing the pet (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['placeOrder'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\Order
      */
-    public function placeOrder($order)
+    public function placeOrder($order, string $contentType = self::contentTypes['placeOrder'][0])
     {
-        list($response) = $this->placeOrderWithHttpInfo($order);
+        list($response) = $this->placeOrderWithHttpInfo($order, $contentType);
         return $response;
     }
 
@@ -922,14 +941,15 @@ class StoreApi
      * Place an order for a pet
      *
      * @param  \OpenAPI\Client\Model\Order $order order placed for purchasing the pet (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['placeOrder'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\Order, HTTP status code, HTTP response headers (array of strings)
      */
-    public function placeOrderWithHttpInfo($order)
+    public function placeOrderWithHttpInfo($order, string $contentType = self::contentTypes['placeOrder'][0])
     {
-        $request = $this->placeOrderRequest($order);
+        $request = $this->placeOrderRequest($order, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1021,13 +1041,14 @@ class StoreApi
      * Place an order for a pet
      *
      * @param  \OpenAPI\Client\Model\Order $order order placed for purchasing the pet (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['placeOrder'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function placeOrderAsync($order)
+    public function placeOrderAsync($order, string $contentType = self::contentTypes['placeOrder'][0])
     {
-        return $this->placeOrderAsyncWithHttpInfo($order)
+        return $this->placeOrderAsyncWithHttpInfo($order, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1041,14 +1062,15 @@ class StoreApi
      * Place an order for a pet
      *
      * @param  \OpenAPI\Client\Model\Order $order order placed for purchasing the pet (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['placeOrder'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function placeOrderAsyncWithHttpInfo($order)
+    public function placeOrderAsyncWithHttpInfo($order, string $contentType = self::contentTypes['placeOrder'][0])
     {
         $returnType = '\OpenAPI\Client\Model\Order';
-        $request = $this->placeOrderRequest($order);
+        $request = $this->placeOrderRequest($order, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1090,11 +1112,12 @@ class StoreApi
      * Create request for operation 'placeOrder'
      *
      * @param  \OpenAPI\Client\Model\Order $order order placed for purchasing the pet (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['placeOrder'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function placeOrderRequest($order)
+    public function placeOrderRequest($order, string $contentType = self::contentTypes['placeOrder'][0])
     {
 
         // verify the required parameter 'order' is set
@@ -1104,6 +1127,7 @@ class StoreApi
             );
         }
 
+
         $resourcePath = '/store/order';
         $formParams = [];
         $queryParams = [];
@@ -1115,20 +1139,16 @@ class StoreApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/xml', 'application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/xml', 'application/json'],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/xml', 'application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($order)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($order));
             } else {
                 $httpBody = $order;
@@ -1148,9 +1168,9 @@ class StoreApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
index 3fe21fa7064..782f8b811d1 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
@@ -69,7 +69,35 @@ class UserApi
      */
     protected $hostIndex;
 
-    /**
+    /** @var string[] $contentTypes **/
+    public const contentTypes = [
+        'createUser' => [
+            'application/json',
+        ],
+        'createUsersWithArrayInput' => [
+            'application/json',
+        ],
+        'createUsersWithListInput' => [
+            'application/json',
+        ],
+        'deleteUser' => [
+            'application/json',
+        ],
+        'getUserByName' => [
+            'application/json',
+        ],
+        'loginUser' => [
+            'application/json',
+        ],
+        'logoutUser' => [
+            'application/json',
+        ],
+        'updateUser' => [
+            'application/json',
+        ],
+    ];
+
+/**
      * @param ClientInterface $client
      * @param Configuration   $config
      * @param HeaderSelector  $selector
@@ -121,14 +149,15 @@ class UserApi
      * Create user
      *
      * @param  \OpenAPI\Client\Model\User $user Created user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function createUser($user)
+    public function createUser($user, string $contentType = self::contentTypes['createUser'][0])
     {
-        $this->createUserWithHttpInfo($user);
+        $this->createUserWithHttpInfo($user, $contentType);
     }
 
     /**
@@ -137,14 +166,15 @@ class UserApi
      * Create user
      *
      * @param  \OpenAPI\Client\Model\User $user Created user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function createUserWithHttpInfo($user)
+    public function createUserWithHttpInfo($user, string $contentType = self::contentTypes['createUser'][0])
     {
-        $request = $this->createUserRequest($user);
+        $request = $this->createUserRequest($user, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -196,13 +226,14 @@ class UserApi
      * Create user
      *
      * @param  \OpenAPI\Client\Model\User $user Created user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function createUserAsync($user)
+    public function createUserAsync($user, string $contentType = self::contentTypes['createUser'][0])
     {
-        return $this->createUserAsyncWithHttpInfo($user)
+        return $this->createUserAsyncWithHttpInfo($user, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -216,14 +247,15 @@ class UserApi
      * Create user
      *
      * @param  \OpenAPI\Client\Model\User $user Created user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function createUserAsyncWithHttpInfo($user)
+    public function createUserAsyncWithHttpInfo($user, string $contentType = self::contentTypes['createUser'][0])
     {
         $returnType = '';
-        $request = $this->createUserRequest($user);
+        $request = $this->createUserRequest($user, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -252,11 +284,12 @@ class UserApi
      * Create request for operation 'createUser'
      *
      * @param  \OpenAPI\Client\Model\User $user Created user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function createUserRequest($user)
+    public function createUserRequest($user, string $contentType = self::contentTypes['createUser'][0])
     {
 
         // verify the required parameter 'user' is set
@@ -266,6 +299,7 @@ class UserApi
             );
         }
 
+
         $resourcePath = '/user';
         $formParams = [];
         $queryParams = [];
@@ -277,20 +311,16 @@ class UserApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($user)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($user));
             } else {
                 $httpBody = $user;
@@ -310,9 +340,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -347,14 +377,15 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithArrayInput'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function createUsersWithArrayInput($user)
+    public function createUsersWithArrayInput($user, string $contentType = self::contentTypes['createUsersWithArrayInput'][0])
     {
-        $this->createUsersWithArrayInputWithHttpInfo($user);
+        $this->createUsersWithArrayInputWithHttpInfo($user, $contentType);
     }
 
     /**
@@ -363,14 +394,15 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithArrayInput'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function createUsersWithArrayInputWithHttpInfo($user)
+    public function createUsersWithArrayInputWithHttpInfo($user, string $contentType = self::contentTypes['createUsersWithArrayInput'][0])
     {
-        $request = $this->createUsersWithArrayInputRequest($user);
+        $request = $this->createUsersWithArrayInputRequest($user, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -422,13 +454,14 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithArrayInput'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function createUsersWithArrayInputAsync($user)
+    public function createUsersWithArrayInputAsync($user, string $contentType = self::contentTypes['createUsersWithArrayInput'][0])
     {
-        return $this->createUsersWithArrayInputAsyncWithHttpInfo($user)
+        return $this->createUsersWithArrayInputAsyncWithHttpInfo($user, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -442,14 +475,15 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithArrayInput'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function createUsersWithArrayInputAsyncWithHttpInfo($user)
+    public function createUsersWithArrayInputAsyncWithHttpInfo($user, string $contentType = self::contentTypes['createUsersWithArrayInput'][0])
     {
         $returnType = '';
-        $request = $this->createUsersWithArrayInputRequest($user);
+        $request = $this->createUsersWithArrayInputRequest($user, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -478,11 +512,12 @@ class UserApi
      * Create request for operation 'createUsersWithArrayInput'
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithArrayInput'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function createUsersWithArrayInputRequest($user)
+    public function createUsersWithArrayInputRequest($user, string $contentType = self::contentTypes['createUsersWithArrayInput'][0])
     {
 
         // verify the required parameter 'user' is set
@@ -492,6 +527,7 @@ class UserApi
             );
         }
 
+
         $resourcePath = '/user/createWithArray';
         $formParams = [];
         $queryParams = [];
@@ -503,20 +539,16 @@ class UserApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($user)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($user));
             } else {
                 $httpBody = $user;
@@ -536,9 +568,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -573,14 +605,15 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithListInput'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function createUsersWithListInput($user)
+    public function createUsersWithListInput($user, string $contentType = self::contentTypes['createUsersWithListInput'][0])
     {
-        $this->createUsersWithListInputWithHttpInfo($user);
+        $this->createUsersWithListInputWithHttpInfo($user, $contentType);
     }
 
     /**
@@ -589,14 +622,15 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithListInput'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function createUsersWithListInputWithHttpInfo($user)
+    public function createUsersWithListInputWithHttpInfo($user, string $contentType = self::contentTypes['createUsersWithListInput'][0])
     {
-        $request = $this->createUsersWithListInputRequest($user);
+        $request = $this->createUsersWithListInputRequest($user, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -648,13 +682,14 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithListInput'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function createUsersWithListInputAsync($user)
+    public function createUsersWithListInputAsync($user, string $contentType = self::contentTypes['createUsersWithListInput'][0])
     {
-        return $this->createUsersWithListInputAsyncWithHttpInfo($user)
+        return $this->createUsersWithListInputAsyncWithHttpInfo($user, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -668,14 +703,15 @@ class UserApi
      * Creates list of users with given input array
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithListInput'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function createUsersWithListInputAsyncWithHttpInfo($user)
+    public function createUsersWithListInputAsyncWithHttpInfo($user, string $contentType = self::contentTypes['createUsersWithListInput'][0])
     {
         $returnType = '';
-        $request = $this->createUsersWithListInputRequest($user);
+        $request = $this->createUsersWithListInputRequest($user, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -704,11 +740,12 @@ class UserApi
      * Create request for operation 'createUsersWithListInput'
      *
      * @param  \OpenAPI\Client\Model\User[] $user List of user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['createUsersWithListInput'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function createUsersWithListInputRequest($user)
+    public function createUsersWithListInputRequest($user, string $contentType = self::contentTypes['createUsersWithListInput'][0])
     {
 
         // verify the required parameter 'user' is set
@@ -718,6 +755,7 @@ class UserApi
             );
         }
 
+
         $resourcePath = '/user/createWithList';
         $formParams = [];
         $queryParams = [];
@@ -729,20 +767,16 @@ class UserApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($user)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($user));
             } else {
                 $httpBody = $user;
@@ -762,9 +796,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -799,14 +833,15 @@ class UserApi
      * Delete user
      *
      * @param  string $username The name that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function deleteUser($username)
+    public function deleteUser($username, string $contentType = self::contentTypes['deleteUser'][0])
     {
-        $this->deleteUserWithHttpInfo($username);
+        $this->deleteUserWithHttpInfo($username, $contentType);
     }
 
     /**
@@ -815,14 +850,15 @@ class UserApi
      * Delete user
      *
      * @param  string $username The name that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function deleteUserWithHttpInfo($username)
+    public function deleteUserWithHttpInfo($username, string $contentType = self::contentTypes['deleteUser'][0])
     {
-        $request = $this->deleteUserRequest($username);
+        $request = $this->deleteUserRequest($username, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -874,13 +910,14 @@ class UserApi
      * Delete user
      *
      * @param  string $username The name that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function deleteUserAsync($username)
+    public function deleteUserAsync($username, string $contentType = self::contentTypes['deleteUser'][0])
     {
-        return $this->deleteUserAsyncWithHttpInfo($username)
+        return $this->deleteUserAsyncWithHttpInfo($username, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -894,14 +931,15 @@ class UserApi
      * Delete user
      *
      * @param  string $username The name that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function deleteUserAsyncWithHttpInfo($username)
+    public function deleteUserAsyncWithHttpInfo($username, string $contentType = self::contentTypes['deleteUser'][0])
     {
         $returnType = '';
-        $request = $this->deleteUserRequest($username);
+        $request = $this->deleteUserRequest($username, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -930,11 +968,12 @@ class UserApi
      * Create request for operation 'deleteUser'
      *
      * @param  string $username The name that needs to be deleted (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['deleteUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function deleteUserRequest($username)
+    public function deleteUserRequest($username, string $contentType = self::contentTypes['deleteUser'][0])
     {
 
         // verify the required parameter 'username' is set
@@ -944,6 +983,7 @@ class UserApi
             );
         }
 
+
         $resourcePath = '/user/{username}';
         $formParams = [];
         $queryParams = [];
@@ -963,16 +1003,11 @@ class UserApi
         }
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -990,9 +1025,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1027,14 +1062,15 @@ class UserApi
      * Get user by user name
      *
      * @param  string $username The name that needs to be fetched. Use user1 for testing. (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getUserByName'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return \OpenAPI\Client\Model\User
      */
-    public function getUserByName($username)
+    public function getUserByName($username, string $contentType = self::contentTypes['getUserByName'][0])
     {
-        list($response) = $this->getUserByNameWithHttpInfo($username);
+        list($response) = $this->getUserByNameWithHttpInfo($username, $contentType);
         return $response;
     }
 
@@ -1044,14 +1080,15 @@ class UserApi
      * Get user by user name
      *
      * @param  string $username The name that needs to be fetched. Use user1 for testing. (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getUserByName'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of \OpenAPI\Client\Model\User, HTTP status code, HTTP response headers (array of strings)
      */
-    public function getUserByNameWithHttpInfo($username)
+    public function getUserByNameWithHttpInfo($username, string $contentType = self::contentTypes['getUserByName'][0])
     {
-        $request = $this->getUserByNameRequest($username);
+        $request = $this->getUserByNameRequest($username, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1143,13 +1180,14 @@ class UserApi
      * Get user by user name
      *
      * @param  string $username The name that needs to be fetched. Use user1 for testing. (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getUserByName'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getUserByNameAsync($username)
+    public function getUserByNameAsync($username, string $contentType = self::contentTypes['getUserByName'][0])
     {
-        return $this->getUserByNameAsyncWithHttpInfo($username)
+        return $this->getUserByNameAsyncWithHttpInfo($username, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1163,14 +1201,15 @@ class UserApi
      * Get user by user name
      *
      * @param  string $username The name that needs to be fetched. Use user1 for testing. (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getUserByName'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function getUserByNameAsyncWithHttpInfo($username)
+    public function getUserByNameAsyncWithHttpInfo($username, string $contentType = self::contentTypes['getUserByName'][0])
     {
         $returnType = '\OpenAPI\Client\Model\User';
-        $request = $this->getUserByNameRequest($username);
+        $request = $this->getUserByNameRequest($username, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1212,11 +1251,12 @@ class UserApi
      * Create request for operation 'getUserByName'
      *
      * @param  string $username The name that needs to be fetched. Use user1 for testing. (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['getUserByName'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function getUserByNameRequest($username)
+    public function getUserByNameRequest($username, string $contentType = self::contentTypes['getUserByName'][0])
     {
 
         // verify the required parameter 'username' is set
@@ -1226,6 +1266,7 @@ class UserApi
             );
         }
 
+
         $resourcePath = '/user/{username}';
         $formParams = [];
         $queryParams = [];
@@ -1245,16 +1286,11 @@ class UserApi
         }
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/xml', 'application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/xml', 'application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/xml', 'application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -1272,9 +1308,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1310,14 +1346,15 @@ class UserApi
      *
      * @param  string $username The user name for login (required)
      * @param  string $password The password for login in clear text (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['loginUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return string
      */
-    public function loginUser($username, $password)
+    public function loginUser($username, $password, string $contentType = self::contentTypes['loginUser'][0])
     {
-        list($response) = $this->loginUserWithHttpInfo($username, $password);
+        list($response) = $this->loginUserWithHttpInfo($username, $password, $contentType);
         return $response;
     }
 
@@ -1328,14 +1365,15 @@ class UserApi
      *
      * @param  string $username The user name for login (required)
      * @param  string $password The password for login in clear text (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['loginUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of string, HTTP status code, HTTP response headers (array of strings)
      */
-    public function loginUserWithHttpInfo($username, $password)
+    public function loginUserWithHttpInfo($username, $password, string $contentType = self::contentTypes['loginUser'][0])
     {
-        $request = $this->loginUserRequest($username, $password);
+        $request = $this->loginUserRequest($username, $password, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1428,13 +1466,14 @@ class UserApi
      *
      * @param  string $username The user name for login (required)
      * @param  string $password The password for login in clear text (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['loginUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function loginUserAsync($username, $password)
+    public function loginUserAsync($username, $password, string $contentType = self::contentTypes['loginUser'][0])
     {
-        return $this->loginUserAsyncWithHttpInfo($username, $password)
+        return $this->loginUserAsyncWithHttpInfo($username, $password, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1449,14 +1488,15 @@ class UserApi
      *
      * @param  string $username The user name for login (required)
      * @param  string $password The password for login in clear text (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['loginUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function loginUserAsyncWithHttpInfo($username, $password)
+    public function loginUserAsyncWithHttpInfo($username, $password, string $contentType = self::contentTypes['loginUser'][0])
     {
         $returnType = 'string';
-        $request = $this->loginUserRequest($username, $password);
+        $request = $this->loginUserRequest($username, $password, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1499,11 +1539,12 @@ class UserApi
      *
      * @param  string $username The user name for login (required)
      * @param  string $password The password for login in clear text (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['loginUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function loginUserRequest($username, $password)
+    public function loginUserRequest($username, $password, string $contentType = self::contentTypes['loginUser'][0])
     {
 
         // verify the required parameter 'username' is set
@@ -1520,6 +1561,7 @@ class UserApi
             );
         }
 
+
         $resourcePath = '/user/login';
         $formParams = [];
         $queryParams = [];
@@ -1549,16 +1591,11 @@ class UserApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                ['application/xml', 'application/json']
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                ['application/xml', 'application/json'],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/xml', 'application/json', ],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -1576,9 +1613,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1612,14 +1649,15 @@ class UserApi
      *
      * Logs out current logged in user session
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['logoutUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function logoutUser()
+    public function logoutUser(string $contentType = self::contentTypes['logoutUser'][0])
     {
-        $this->logoutUserWithHttpInfo();
+        $this->logoutUserWithHttpInfo($contentType);
     }
 
     /**
@@ -1627,14 +1665,15 @@ class UserApi
      *
      * Logs out current logged in user session
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['logoutUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function logoutUserWithHttpInfo()
+    public function logoutUserWithHttpInfo(string $contentType = self::contentTypes['logoutUser'][0])
     {
-        $request = $this->logoutUserRequest();
+        $request = $this->logoutUserRequest($contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1685,13 +1724,14 @@ class UserApi
      *
      * Logs out current logged in user session
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['logoutUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function logoutUserAsync()
+    public function logoutUserAsync(string $contentType = self::contentTypes['logoutUser'][0])
     {
-        return $this->logoutUserAsyncWithHttpInfo()
+        return $this->logoutUserAsyncWithHttpInfo($contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1704,14 +1744,15 @@ class UserApi
      *
      * Logs out current logged in user session
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['logoutUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function logoutUserAsyncWithHttpInfo()
+    public function logoutUserAsyncWithHttpInfo(string $contentType = self::contentTypes['logoutUser'][0])
     {
         $returnType = '';
-        $request = $this->logoutUserRequest();
+        $request = $this->logoutUserRequest($contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1739,13 +1780,15 @@ class UserApi
     /**
      * Create request for operation 'logoutUser'
      *
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['logoutUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function logoutUserRequest()
+    public function logoutUserRequest(string $contentType = self::contentTypes['logoutUser'][0])
     {
 
+
         $resourcePath = '/user/logout';
         $formParams = [];
         $queryParams = [];
@@ -1757,16 +1800,11 @@ class UserApi
 
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                []
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (count($formParams) > 0) {
@@ -1784,9 +1822,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
@@ -1822,14 +1860,15 @@ class UserApi
      *
      * @param  string $username name that need to be deleted (required)
      * @param  \OpenAPI\Client\Model\User $user Updated user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updateUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return void
      */
-    public function updateUser($username, $user)
+    public function updateUser($username, $user, string $contentType = self::contentTypes['updateUser'][0])
     {
-        $this->updateUserWithHttpInfo($username, $user);
+        $this->updateUserWithHttpInfo($username, $user, $contentType);
     }
 
     /**
@@ -1839,14 +1878,15 @@ class UserApi
      *
      * @param  string $username name that need to be deleted (required)
      * @param  \OpenAPI\Client\Model\User $user Updated user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updateUser'] to see the possible values for this operation
      *
      * @throws \OpenAPI\Client\ApiException on non-2xx response
      * @throws \InvalidArgumentException
      * @return array of null, HTTP status code, HTTP response headers (array of strings)
      */
-    public function updateUserWithHttpInfo($username, $user)
+    public function updateUserWithHttpInfo($username, $user, string $contentType = self::contentTypes['updateUser'][0])
     {
-        $request = $this->updateUserRequest($username, $user);
+        $request = $this->updateUserRequest($username, $user, $contentType);
 
         try {
             $options = $this->createHttpClientOption();
@@ -1899,13 +1939,14 @@ class UserApi
      *
      * @param  string $username name that need to be deleted (required)
      * @param  \OpenAPI\Client\Model\User $user Updated user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updateUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function updateUserAsync($username, $user)
+    public function updateUserAsync($username, $user, string $contentType = self::contentTypes['updateUser'][0])
     {
-        return $this->updateUserAsyncWithHttpInfo($username, $user)
+        return $this->updateUserAsyncWithHttpInfo($username, $user, $contentType)
             ->then(
                 function ($response) {
                     return $response[0];
@@ -1920,14 +1961,15 @@ class UserApi
      *
      * @param  string $username name that need to be deleted (required)
      * @param  \OpenAPI\Client\Model\User $user Updated user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updateUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Promise\PromiseInterface
      */
-    public function updateUserAsyncWithHttpInfo($username, $user)
+    public function updateUserAsyncWithHttpInfo($username, $user, string $contentType = self::contentTypes['updateUser'][0])
     {
         $returnType = '';
-        $request = $this->updateUserRequest($username, $user);
+        $request = $this->updateUserRequest($username, $user, $contentType);
 
         return $this->client
             ->sendAsync($request, $this->createHttpClientOption())
@@ -1957,11 +1999,12 @@ class UserApi
      *
      * @param  string $username name that need to be deleted (required)
      * @param  \OpenAPI\Client\Model\User $user Updated user object (required)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['updateUser'] to see the possible values for this operation
      *
      * @throws \InvalidArgumentException
      * @return \GuzzleHttp\Psr7\Request
      */
-    public function updateUserRequest($username, $user)
+    public function updateUserRequest($username, $user, string $contentType = self::contentTypes['updateUser'][0])
     {
 
         // verify the required parameter 'username' is set
@@ -1978,6 +2021,7 @@ class UserApi
             );
         }
 
+
         $resourcePath = '/user/{username}';
         $formParams = [];
         $queryParams = [];
@@ -1997,20 +2041,16 @@ class UserApi
         }
 
 
-        if ($multipart) {
-            $headers = $this->headerSelector->selectHeadersForMultipart(
-                []
-            );
-        } else {
-            $headers = $this->headerSelector->selectHeaders(
-                [],
-                ['application/json']
-            );
-        }
+        $headers = $this->headerSelector->selectHeaders(
+            [],
+            $contentType,
+            $multipart
+        );
 
         // for model (json/xml)
         if (isset($user)) {
-            if ($headers['Content-Type'] === 'application/json') {
+            if (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the body
                 $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($user));
             } else {
                 $httpBody = $user;
@@ -2030,9 +2070,9 @@ class UserApi
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
                 $httpBody = \GuzzleHttp\json_encode($formParams);
-
             } else {
                 // for HTTP post (form)
                 $httpBody = ObjectSerializer::buildQuery($formParams);
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/HeaderSelector.php b/samples/client/petstore/php/OpenAPIClient-php/lib/HeaderSelector.php
index d3a56bae3c3..769fdb8d0c1 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/HeaderSelector.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/HeaderSelector.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * ApiException
+ * HeaderSelector
  * PHP version 7.4
  *
  * @category Class
@@ -27,10 +27,8 @@
 
 namespace OpenAPI\Client;
 
-use \Exception;
-
 /**
- * ApiException Class Doc Comment
+ * HeaderSelector Class Doc Comment
  *
  * @category Class
  * @package  OpenAPI\Client
@@ -41,10 +39,11 @@ class HeaderSelector
 {
     /**
      * @param string[] $accept
-     * @param string[] $contentTypes
-     * @return array
+     * @param string   $contentType
+     * @param bool     $isMultipart
+     * @return string[]
      */
-    public function selectHeaders($accept, $contentTypes)
+    public function selectHeaders(array $accept, string $contentType, bool $isMultipart): array
     {
         $headers = [];
 
@@ -52,20 +51,13 @@ class HeaderSelector
         if ($accept !== null) {
             $headers['Accept'] = $accept;
         }
+        if(!$isMultipart) {
+            if($contentType === '') {
+                $contentType = 'application/json';
+            }
+            $headers['Content-Type'] = $contentType;
+        }
 
-        $headers['Content-Type'] = $this->selectContentTypeHeader($contentTypes);
-        return $headers;
-    }
-
-    /**
-     * @param string[] $accept
-     * @return array
-     */
-    public function selectHeadersForMultipart($accept)
-    {
-        $headers = $this->selectHeaders($accept, []);
-
-        unset($headers['Content-Type']);
         return $headers;
     }
 
@@ -86,22 +78,4 @@ class HeaderSelector
             return implode(',', $accept);
         }
     }
-
-    /**
-     * Return the content type based on an array of content-type provided
-     *
-     * @param string[] $contentType Array fo content-type
-     *
-     * @return string Content-Type (e.g. application/json)
-     */
-    private function selectContentTypeHeader($contentType)
-    {
-        if (count($contentType) === 0 || (count($contentType) === 1 && $contentType[0] === '')) {
-            return 'application/json';
-        } elseif (preg_grep("/application\/json/i", $contentType)) {
-            return 'application/json';
-        } else {
-            return implode(',', $contentType);
-        }
-    }
 }
diff --git a/samples/client/petstore/php/OpenAPIClient-php/tests/HeaderSelectorTest.php b/samples/client/petstore/php/OpenAPIClient-php/tests/HeaderSelectorTest.php
index 6dd19a675f6..bbc54c5a78f 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/tests/HeaderSelectorTest.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/tests/HeaderSelectorTest.php
@@ -1,58 +1,70 @@
 <?php
-
 namespace OpenAPI\Client;
 
 use PHPUnit\Framework\TestCase;
 
+/**
+ * class HeaderSelectorTest
+ *
+ * @package OpenAPI\Client
+ */
 class HeaderSelectorTest extends TestCase
 {
-    public function testSelectingHeaders()
+    /**
+     * @dataProvider headersProvider
+     * @param string[] $accept
+     * @param string   $contentType
+     * @param bool     $isMultiple
+     * @param string[] $expectedHeaders
+     */
+    public function testSelectHeaders(array $accept, string $contentType, bool $isMultiple, array $expectedHeaders): void
     {
         $selector = new HeaderSelector();
-        $headers = $selector->selectHeaders([
-            'application/xml',
-            'application/json'
-        ], []);
-        $this->assertSame('application/json', $headers['Accept']);
 
-        $headers = $selector->selectHeaders([], []);
-        $this->assertArrayNotHasKey('Accept', $headers);
-
-        $header = $selector->selectHeaders([
-            'application/yaml',
-            'application/xml'
-        ], []);
-        $this->assertSame('application/yaml,application/xml', $header['Accept']);
-
-        // test selectHeaderContentType
-        $headers = $selector->selectHeaders([], [
-            'application/xml',
-            'application/json'
-        ]);
-        $this->assertSame('application/json', $headers['Content-Type']);
-
-        $headers = $selector->selectHeaders([], []);
-        $this->assertSame('application/json', $headers['Content-Type']);
-        $headers = $selector->selectHeaders([], [
-            'application/yaml',
-            'application/xml'
-        ]);
-        $this->assertSame('application/yaml,application/xml', $headers['Content-Type']);
+        $headers = $selector->selectHeaders($accept, $contentType, $isMultiple);
+        $this->assertEquals($expectedHeaders, $headers);
     }
 
-    public function testSelectingHeadersForMultipartBody()
+    /**
+     * @return array[][]
+     */
+    public function headersProvider(): array
     {
-        // test selectHeaderAccept
-        $selector = new HeaderSelector();
-        $headers = $selector->selectHeadersForMultipart([
-            'application/xml',
-            'application/json'
-        ]);
-        $this->assertSame('application/json', $headers['Accept']);
-        $this->assertArrayNotHasKey('Content-Type', $headers);
-
-        $headers = $selector->selectHeadersForMultipart([]);
-        $this->assertArrayNotHasKey('Accept', $headers);
-        $this->assertArrayNotHasKey('Content-Type', $headers);
+        return [
+            // array $accept, string $contentType, bool $isMultipart, array $expectedHeaders
+            [
+                [], 'application/xml', false, ['Content-Type' => 'application/xml'],
+            ],
+            [
+                [], 'application/xml', true, [],
+            ],
+            [
+                ['application/xml'], '', false, ['Accept' => 'application/xml', 'Content-Type' => 'application/json'],
+            ],
+            [
+                ['application/xml'], '', true, ['Accept' => 'application/xml'],
+            ],
+            [
+                ['application/xml'], 'application/xml', false, ['Accept' => 'application/xml', 'Content-Type' => 'application/xml'],
+            ],
+            [
+                ['application/xml'], 'application/xml', true, ['Accept' => 'application/xml'],
+            ],
+            [
+                ['application/xml', 'text/html'], 'application/xml', false, ['Accept' => 'application/xml,text/html', 'Content-Type' => 'application/xml'],
+            ],
+            [
+                ['application/json', 'text/html'], 'application/xml', false, ['Accept' => 'application/json', 'Content-Type' => 'application/xml'],
+            ],
+            [
+                ['text/html', 'application/json'], 'application/xml', false, ['Accept' => 'application/json', 'Content-Type' => 'application/xml'],
+            ],
+            [
+                ['application/json;format=flowed'], 'text/plain;format=fixed', false, ['Accept' => 'application/json;format=flowed', 'Content-Type' => 'text/plain;format=fixed'],
+            ],
+            [
+                ['text/html', 'application/json;format=flowed'], 'text/plain;format=fixed', false, ['Accept' => 'application/json;format=flowed', 'Content-Type' => 'text/plain;format=fixed'],
+            ],
+        ];
     }
 }