diff --git a/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache b/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache
index 4317aaf327a..81dd8a50387 100644
--- a/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache
+++ b/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache
@@ -18,6 +18,7 @@
 
 namespace {{invokerPackage}};
 
+use ArrayAccess;
 use DateTimeInterface;
 use DateTime;
 use GuzzleHttp\Psr7\Utils;
@@ -316,20 +317,33 @@ class ObjectSerializer
     }
 
     /**
-     * Take value and turn it into a string suitable for inclusion in
+     * Take value and turn it into an array suitable for inclusion in
      * the http body (form parameter). If it's a string, pass through unchanged
      * If it's a datetime object, format it in ISO8601
      *
-     * @param string|\SplFileObject $value the value of the form parameter
+     * @param string|bool|array|DateTime|ArrayAccess|\SplFileObject $value the value of the form parameter
      *
-     * @return string the form string
+     * @return array [key => value] of formdata
      */
-    public static function toFormValue(string|\SplFileObject $value): string
-    {
+    public static function toFormValue(
+        string $key,
+        string|bool|array|DateTime|ArrayAccess|\SplFileObject $value,
+    ): array {
         if ($value instanceof \SplFileObject) {
-            return $value->getRealPath();
+            return [$key => $value->getRealPath()];
+        } elseif (is_array($value) || $value instanceof ArrayAccess) {
+            $flattened = [];
+            $result = [];
+
+            self::flattenArray(json_decode(json_encode($value), true), $flattened);
+
+            foreach ($flattened as $k => $v) {
+                $result["{$key}{$k}"] = self::toString($v);
+            }
+
+            return $result;
         } else {
-            return self::toString($value);
+            return [$key => self::toString($value)];
         }
     }
 
@@ -598,4 +612,58 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * credit: https://github.com/FranBar1966/FlatPHP
+     */
+    private static function flattenArray(
+        ArrayAccess|array $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flattenArray($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }
diff --git a/modules/openapi-generator/src/main/resources/php-nextgen/api.mustache b/modules/openapi-generator/src/main/resources/php-nextgen/api.mustache
index 7d679de68df..96c394a31bb 100644
--- a/modules/openapi-generator/src/main/resources/php-nextgen/api.mustache
+++ b/modules/openapi-generator/src/main/resources/php-nextgen/api.mustache
@@ -759,13 +759,13 @@ use {{invokerPackage}}\ObjectSerializer;
                 $formParams['{{baseName}}'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('{{baseName}}', $paramFile)['{{baseName}}'],
                         'rb'
                     );
             }
             {{/isFile}}
             {{^isFile}}
-            $formParams['{{baseName}}'] = ObjectSerializer::toFormValue(${{paramName}});
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('{{baseName}}', ${{paramName}}));
             {{/isFile}}
         }
         {{/formParams}}
diff --git a/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache b/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
index 7c2a64ee47b..e25b25ce28d 100644
--- a/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
+++ b/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
@@ -19,6 +19,7 @@
 
 namespace {{invokerPackage}};
 
+use ArrayAccess;
 use GuzzleHttp\Psr7\Utils;
 use {{modelPackage}}\ModelInterface;
 
@@ -315,20 +316,31 @@ class ObjectSerializer
     }
 
     /**
-     * Take value and turn it into a string suitable for inclusion in
+     * Take value and turn it into an array suitable for inclusion in
      * the http body (form parameter). If it's a string, pass through unchanged
      * If it's a datetime object, format it in ISO8601
      *
-     * @param string|\SplFileObject $value the value of the form parameter
+     * @param string|bool|array|DateTime|ArrayAccess|\SplFileObject $value the value of the form parameter
      *
-     * @return string the form string
+     * @return array [key => value] of formdata
      */
-    public static function toFormValue($value)
+    public static function toFormValue(string $key, mixed $value)
     {
         if ($value instanceof \SplFileObject) {
-            return $value->getRealPath();
+            return [$key => $value->getRealPath()];
+        } elseif (is_array($value) || $value instanceof ArrayAccess) {
+            $flattened = [];
+            $result = [];
+
+            self::flattenArray(json_decode(json_encode($value), true), $flattened);
+
+            foreach ($flattened as $k => $v) {
+                $result["{$key}{$k}"] = self::toString($v);
+            }
+
+            return $result;
         } else {
-            return self::toString($value);
+            return [$key => self::toString($value)];
         }
     }
 
@@ -605,4 +617,81 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * @param \ArrayAccess|array $source
+     *
+     * credit: https://github.com/FranBar1966/FlatPHP
+     */
+    private static function flattenArray(
+        mixed $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        /**
+         * array_is_list only in PHP >= 8.1
+         *
+         * credit: https://www.php.net/manual/en/function.array-is-list.php#127044
+         */
+        if (!function_exists('array_is_list')) {
+            function array_is_list(array $array)
+            {
+                $i = -1;
+
+                foreach ($array as $k => $v) {
+                    ++$i;
+                    if ($k !== $i) {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flattenArray($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }
diff --git a/modules/openapi-generator/src/main/resources/php/api.mustache b/modules/openapi-generator/src/main/resources/php/api.mustache
index fedb7d53241..f6246863be4 100644
--- a/modules/openapi-generator/src/main/resources/php/api.mustache
+++ b/modules/openapi-generator/src/main/resources/php/api.mustache
@@ -677,13 +677,13 @@ use {{invokerPackage}}\ObjectSerializer;
             $paramFiles = is_array(${{paramName}}) ? ${{paramName}} : [${{paramName}}];
             foreach ($paramFiles as $paramFile) {
                 $formParams['{{baseName}}'][] = \GuzzleHttp\Psr7\Utils::tryFopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('{{baseName}}', $paramFile)['{{baseName}}'],
                     'rb'
                 );
             }
             {{/isFile}}
             {{^isFile}}
-            $formParams['{{baseName}}'] = ObjectSerializer::toFormValue(${{paramName}});
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('{{baseName}}', ${{paramName}}));
             {{/isFile}}
         }
         {{/formParams}}
diff --git a/modules/openapi-generator/src/main/resources/php/libraries/psr-18/api.mustache b/modules/openapi-generator/src/main/resources/php/libraries/psr-18/api.mustache
index 7b286d94d03..16e76da9074 100644
--- a/modules/openapi-generator/src/main/resources/php/libraries/psr-18/api.mustache
+++ b/modules/openapi-generator/src/main/resources/php/libraries/psr-18/api.mustache
@@ -593,13 +593,13 @@ use function sprintf;
             $paramFiles = is_array(${{paramName}}) ? ${{paramName}} : [${{paramName}}];
             foreach ($paramFiles as $paramFile) {
                 $formParams['{{baseName}}'][] = \GuzzleHttp\Psr7\try_fopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('{{baseName}}', $paramFile)['{{baseName}}'],
                     'rb'
                 );
             }
             {{/isFile}}
             {{^isFile}}
-            $formParams['{{baseName}}'] = ObjectSerializer::toFormValue(${{paramName}});
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('{{baseName}}', ${{paramName}}));
             {{/isFile}}
         }
         {{/formParams}}
diff --git a/modules/openapi-generator/src/test/resources/3_0/php/petstore-with-fake-endpoints-models-for-testing.yaml b/modules/openapi-generator/src/test/resources/3_0/php/petstore-with-fake-endpoints-models-for-testing.yaml
index d9e47ca9ed8..e971a99dc26 100644
--- a/modules/openapi-generator/src/test/resources/3_0/php/petstore-with-fake-endpoints-models-for-testing.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/php/petstore-with-fake-endpoints-models-for-testing.yaml
@@ -311,6 +311,37 @@ paths:
                   description: file to upload
                   type: string
                   format: binary
+  '/pet/{petId}/uploadImageFullFormData':
+    post:
+      tags:
+        - pet
+      summary: uploads an image attached to a Pet object as formdata
+      description: ''
+      operationId: uploadImageFullFormData
+      parameters:
+        - name: petId
+          in: path
+          description: ID of pet to update
+          required: true
+          schema:
+            type: integer
+            format: int64
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ApiResponse'
+      security:
+        - petstore_auth:
+            - 'write:pets'
+            - 'read:pets'
+      requestBody:
+        content:
+          multipart/form-data:
+            schema:
+              $ref: '#/components/schemas/PetWithFile'
   /store/inventory:
     get:
       tags:
@@ -1566,6 +1597,54 @@ components:
             - sold
       xml:
         name: Pet
+    PetWithFile:
+      type: object
+      required:
+        - name
+        - photoUrls
+      properties:
+        id:
+          type: integer
+          format: int64
+          x-is-unique: true
+        category:
+          $ref: '#/components/schemas/Category'
+        name:
+          type: string
+          example: doggie
+        photoUrls:
+          type: array
+          xml:
+            name: photoUrl
+            wrapped: true
+          items:
+            type: string
+          uniqueItems: true
+        tags:
+          type: array
+          xml:
+            name: tag
+            wrapped: true
+          items:
+            $ref: '#/components/schemas/Tag'
+        status:
+          type: string
+          description: pet status in the store
+          enum:
+            - available
+            - pending
+            - sold
+        file:
+          description: file to upload
+          type: string
+          format: binary
+        multiple_files:
+          type: array
+          items:
+            type: string
+            format: binary
+      xml:
+        name: PetWithFile
     ApiResponse:
       type: object
       properties:
diff --git a/samples/client/echo_api/php-nextgen-streaming/src/Api/BodyApi.php b/samples/client/echo_api/php-nextgen-streaming/src/Api/BodyApi.php
index 38d54217fc1..78c0645f1ad 100644
--- a/samples/client/echo_api/php-nextgen-streaming/src/Api/BodyApi.php
+++ b/samples/client/echo_api/php-nextgen-streaming/src/Api/BodyApi.php
@@ -1034,7 +1034,7 @@ class BodyApi
                 $formParams['files'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('files', $paramFile)['files'],
                         'rb'
                     );
             }
@@ -1357,7 +1357,7 @@ class BodyApi
                 $formParams['my-file'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('my-file', $paramFile)['my-file'],
                         'rb'
                     );
             }
diff --git a/samples/client/echo_api/php-nextgen-streaming/src/Api/FormApi.php b/samples/client/echo_api/php-nextgen-streaming/src/Api/FormApi.php
index 2408c27bba4..193033455ac 100644
--- a/samples/client/echo_api/php-nextgen-streaming/src/Api/FormApi.php
+++ b/samples/client/echo_api/php-nextgen-streaming/src/Api/FormApi.php
@@ -408,15 +408,15 @@ class FormApi
 
         // form params
         if ($integer_form !== null) {
-            $formParams['integer_form'] = ObjectSerializer::toFormValue($integer_form);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('integer_form', $integer_form));
         }
         // form params
         if ($boolean_form !== null) {
-            $formParams['boolean_form'] = ObjectSerializer::toFormValue($boolean_form);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('boolean_form', $boolean_form));
         }
         // form params
         if ($string_form !== null) {
-            $formParams['string_form'] = ObjectSerializer::toFormValue($string_form);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('string_form', $string_form));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -735,7 +735,7 @@ class FormApi
 
         // form params
         if ($marker !== null) {
-            $formParams['marker'] = ObjectSerializer::toFormValue($marker);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('marker', $marker));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -1103,27 +1103,27 @@ class FormApi
 
         // form params
         if ($form1 !== null) {
-            $formParams['form1'] = ObjectSerializer::toFormValue($form1);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form1', $form1));
         }
         // form params
         if ($form2 !== null) {
-            $formParams['form2'] = ObjectSerializer::toFormValue($form2);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form2', $form2));
         }
         // form params
         if ($form3 !== null) {
-            $formParams['form3'] = ObjectSerializer::toFormValue($form3);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form3', $form3));
         }
         // form params
         if ($form4 !== null) {
-            $formParams['form4'] = ObjectSerializer::toFormValue($form4);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form4', $form4));
         }
         // form params
         if ($id !== null) {
-            $formParams['id'] = ObjectSerializer::toFormValue($id);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('id', $id));
         }
         // form params
         if ($name !== null) {
-            $formParams['name'] = ObjectSerializer::toFormValue($name);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('name', $name));
         }
 
         $headers = $this->headerSelector->selectHeaders(
diff --git a/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php b/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php
index b4a6f39fa1a..ac7cf8884fd 100644
--- a/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php
+++ b/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php
@@ -28,6 +28,7 @@
 
 namespace OpenAPI\Client;
 
+use ArrayAccess;
 use DateTimeInterface;
 use DateTime;
 use GuzzleHttp\Psr7\Utils;
@@ -326,20 +327,33 @@ class ObjectSerializer
     }
 
     /**
-     * Take value and turn it into a string suitable for inclusion in
+     * Take value and turn it into an array suitable for inclusion in
      * the http body (form parameter). If it's a string, pass through unchanged
      * If it's a datetime object, format it in ISO8601
      *
-     * @param string|\SplFileObject $value the value of the form parameter
+     * @param string|bool|array|DateTime|ArrayAccess|\SplFileObject $value the value of the form parameter
      *
-     * @return string the form string
+     * @return array [key => value] of formdata
      */
-    public static function toFormValue(string|\SplFileObject $value): string
-    {
+    public static function toFormValue(
+        string $key,
+        string|bool|array|DateTime|ArrayAccess|\SplFileObject $value,
+    ): array {
         if ($value instanceof \SplFileObject) {
-            return $value->getRealPath();
+            return [$key => $value->getRealPath()];
+        } elseif (is_array($value) || $value instanceof ArrayAccess) {
+            $flattened = [];
+            $result = [];
+
+            self::flattenArray(json_decode(json_encode($value), true), $flattened);
+
+            foreach ($flattened as $k => $v) {
+                $result["{$key}{$k}"] = self::toString($v);
+            }
+
+            return $result;
         } else {
-            return self::toString($value);
+            return [$key => self::toString($value)];
         }
     }
 
@@ -608,4 +622,58 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * credit: https://github.com/FranBar1966/FlatPHP
+     */
+    private static function flattenArray(
+        ArrayAccess|array $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flattenArray($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }
diff --git a/samples/client/echo_api/php-nextgen/src/Api/BodyApi.php b/samples/client/echo_api/php-nextgen/src/Api/BodyApi.php
index 2232ec257da..c2459f1777e 100644
--- a/samples/client/echo_api/php-nextgen/src/Api/BodyApi.php
+++ b/samples/client/echo_api/php-nextgen/src/Api/BodyApi.php
@@ -1034,7 +1034,7 @@ class BodyApi
                 $formParams['files'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('files', $paramFile)['files'],
                         'rb'
                     );
             }
@@ -1357,7 +1357,7 @@ class BodyApi
                 $formParams['my-file'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('my-file', $paramFile)['my-file'],
                         'rb'
                     );
             }
diff --git a/samples/client/echo_api/php-nextgen/src/Api/FormApi.php b/samples/client/echo_api/php-nextgen/src/Api/FormApi.php
index 2408c27bba4..193033455ac 100644
--- a/samples/client/echo_api/php-nextgen/src/Api/FormApi.php
+++ b/samples/client/echo_api/php-nextgen/src/Api/FormApi.php
@@ -408,15 +408,15 @@ class FormApi
 
         // form params
         if ($integer_form !== null) {
-            $formParams['integer_form'] = ObjectSerializer::toFormValue($integer_form);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('integer_form', $integer_form));
         }
         // form params
         if ($boolean_form !== null) {
-            $formParams['boolean_form'] = ObjectSerializer::toFormValue($boolean_form);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('boolean_form', $boolean_form));
         }
         // form params
         if ($string_form !== null) {
-            $formParams['string_form'] = ObjectSerializer::toFormValue($string_form);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('string_form', $string_form));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -735,7 +735,7 @@ class FormApi
 
         // form params
         if ($marker !== null) {
-            $formParams['marker'] = ObjectSerializer::toFormValue($marker);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('marker', $marker));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -1103,27 +1103,27 @@ class FormApi
 
         // form params
         if ($form1 !== null) {
-            $formParams['form1'] = ObjectSerializer::toFormValue($form1);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form1', $form1));
         }
         // form params
         if ($form2 !== null) {
-            $formParams['form2'] = ObjectSerializer::toFormValue($form2);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form2', $form2));
         }
         // form params
         if ($form3 !== null) {
-            $formParams['form3'] = ObjectSerializer::toFormValue($form3);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form3', $form3));
         }
         // form params
         if ($form4 !== null) {
-            $formParams['form4'] = ObjectSerializer::toFormValue($form4);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('form4', $form4));
         }
         // form params
         if ($id !== null) {
-            $formParams['id'] = ObjectSerializer::toFormValue($id);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('id', $id));
         }
         // form params
         if ($name !== null) {
-            $formParams['name'] = ObjectSerializer::toFormValue($name);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('name', $name));
         }
 
         $headers = $this->headerSelector->selectHeaders(
diff --git a/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php b/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php
index b4a6f39fa1a..ac7cf8884fd 100644
--- a/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php
+++ b/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php
@@ -28,6 +28,7 @@
 
 namespace OpenAPI\Client;
 
+use ArrayAccess;
 use DateTimeInterface;
 use DateTime;
 use GuzzleHttp\Psr7\Utils;
@@ -326,20 +327,33 @@ class ObjectSerializer
     }
 
     /**
-     * Take value and turn it into a string suitable for inclusion in
+     * Take value and turn it into an array suitable for inclusion in
      * the http body (form parameter). If it's a string, pass through unchanged
      * If it's a datetime object, format it in ISO8601
      *
-     * @param string|\SplFileObject $value the value of the form parameter
+     * @param string|bool|array|DateTime|ArrayAccess|\SplFileObject $value the value of the form parameter
      *
-     * @return string the form string
+     * @return array [key => value] of formdata
      */
-    public static function toFormValue(string|\SplFileObject $value): string
-    {
+    public static function toFormValue(
+        string $key,
+        string|bool|array|DateTime|ArrayAccess|\SplFileObject $value,
+    ): array {
         if ($value instanceof \SplFileObject) {
-            return $value->getRealPath();
+            return [$key => $value->getRealPath()];
+        } elseif (is_array($value) || $value instanceof ArrayAccess) {
+            $flattened = [];
+            $result = [];
+
+            self::flattenArray(json_decode(json_encode($value), true), $flattened);
+
+            foreach ($flattened as $k => $v) {
+                $result["{$key}{$k}"] = self::toString($v);
+            }
+
+            return $result;
         } else {
-            return self::toString($value);
+            return [$key => self::toString($value)];
         }
     }
 
@@ -608,4 +622,58 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * credit: https://github.com/FranBar1966/FlatPHP
+     */
+    private static function flattenArray(
+        ArrayAccess|array $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flattenArray($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }
diff --git a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/FakeApi.php b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/FakeApi.php
index 31defa1d789..07ed43c9eaa 100644
--- a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/FakeApi.php
+++ b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/FakeApi.php
@@ -4585,39 +4585,39 @@ class FakeApi
 
         // form params
         if ($integer !== null) {
-            $formParams['integer'] = ObjectSerializer::toFormValue($integer);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('integer', $integer));
         }
         // form params
         if ($int32 !== null) {
-            $formParams['int32'] = ObjectSerializer::toFormValue($int32);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('int32', $int32));
         }
         // form params
         if ($int64 !== null) {
-            $formParams['int64'] = ObjectSerializer::toFormValue($int64);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('int64', $int64));
         }
         // form params
         if ($number !== null) {
-            $formParams['number'] = ObjectSerializer::toFormValue($number);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('number', $number));
         }
         // form params
         if ($float !== null) {
-            $formParams['float'] = ObjectSerializer::toFormValue($float);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('float', $float));
         }
         // form params
         if ($double !== null) {
-            $formParams['double'] = ObjectSerializer::toFormValue($double);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('double', $double));
         }
         // form params
         if ($string !== null) {
-            $formParams['string'] = ObjectSerializer::toFormValue($string);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('string', $string));
         }
         // form params
         if ($pattern_without_delimiter !== null) {
-            $formParams['pattern_without_delimiter'] = ObjectSerializer::toFormValue($pattern_without_delimiter);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('pattern_without_delimiter', $pattern_without_delimiter));
         }
         // form params
         if ($byte !== null) {
-            $formParams['byte'] = ObjectSerializer::toFormValue($byte);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('byte', $byte));
         }
         // form params
         if ($binary !== null) {
@@ -4628,26 +4628,26 @@ class FakeApi
                 $formParams['binary'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('binary', $paramFile)['binary'],
                         'rb'
                     );
             }
         }
         // form params
         if ($date !== null) {
-            $formParams['date'] = ObjectSerializer::toFormValue($date);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('date', $date));
         }
         // form params
         if ($date_time !== null) {
-            $formParams['dateTime'] = ObjectSerializer::toFormValue($date_time);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('dateTime', $date_time));
         }
         // form params
         if ($password !== null) {
-            $formParams['password'] = ObjectSerializer::toFormValue($password);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('password', $password));
         }
         // form params
         if ($callback !== null) {
-            $formParams['callback'] = ObjectSerializer::toFormValue($callback);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('callback', $callback));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -5017,11 +5017,11 @@ class FakeApi
 
         // form params
         if ($enum_form_string_array !== null) {
-            $formParams['enum_form_string_array'] = ObjectSerializer::toFormValue($enum_form_string_array);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('enum_form_string_array', $enum_form_string_array));
         }
         // form params
         if ($enum_form_string !== null) {
-            $formParams['enum_form_string'] = ObjectSerializer::toFormValue($enum_form_string);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('enum_form_string', $enum_form_string));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -6055,11 +6055,11 @@ class FakeApi
 
         // form params
         if ($param !== null) {
-            $formParams['param'] = ObjectSerializer::toFormValue($param);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('param', $param));
         }
         // form params
         if ($param2 !== null) {
-            $formParams['param2'] = ObjectSerializer::toFormValue($param2);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('param2', $param2));
         }
 
         $headers = $this->headerSelector->selectHeaders(
diff --git a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/PetApi.php b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/PetApi.php
index 2ad06b017ec..564d43a7150 100644
--- a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/PetApi.php
+++ b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/PetApi.php
@@ -2364,11 +2364,11 @@ class PetApi
 
         // form params
         if ($name !== null) {
-            $formParams['name'] = ObjectSerializer::toFormValue($name);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('name', $name));
         }
         // form params
         if ($status !== null) {
-            $formParams['status'] = ObjectSerializer::toFormValue($status);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('status', $status));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -2721,7 +2721,7 @@ class PetApi
 
         // form params
         if ($additional_metadata !== null) {
-            $formParams['additionalMetadata'] = ObjectSerializer::toFormValue($additional_metadata);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('additionalMetadata', $additional_metadata));
         }
         // form params
         if ($file !== null) {
@@ -2732,7 +2732,7 @@ class PetApi
                 $formParams['file'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('file', $paramFile)['file'],
                         'rb'
                     );
             }
@@ -3094,7 +3094,7 @@ class PetApi
 
         // form params
         if ($additional_metadata !== null) {
-            $formParams['additionalMetadata'] = ObjectSerializer::toFormValue($additional_metadata);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('additionalMetadata', $additional_metadata));
         }
         // form params
         if ($required_file !== null) {
@@ -3105,7 +3105,7 @@ class PetApi
                 $formParams['requiredFile'][] = $paramFile instanceof \Psr\Http\Message\StreamInterface
                     ? $paramFile
                     : \GuzzleHttp\Psr7\Utils::tryFopen(
-                        ObjectSerializer::toFormValue($paramFile),
+                        ObjectSerializer::toFormValue('requiredFile', $paramFile)['requiredFile'],
                         'rb'
                     );
             }
diff --git a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php
index 6ebd32a22e8..a3f70ac9a98 100644
--- a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php
+++ b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php
@@ -27,6 +27,7 @@
 
 namespace OpenAPI\Client;
 
+use ArrayAccess;
 use DateTimeInterface;
 use DateTime;
 use GuzzleHttp\Psr7\Utils;
@@ -325,20 +326,33 @@ class ObjectSerializer
     }
 
     /**
-     * Take value and turn it into a string suitable for inclusion in
+     * Take value and turn it into an array suitable for inclusion in
      * the http body (form parameter). If it's a string, pass through unchanged
      * If it's a datetime object, format it in ISO8601
      *
-     * @param string|\SplFileObject $value the value of the form parameter
+     * @param string|bool|array|DateTime|ArrayAccess|\SplFileObject $value the value of the form parameter
      *
-     * @return string the form string
+     * @return array [key => value] of formdata
      */
-    public static function toFormValue(string|\SplFileObject $value): string
-    {
+    public static function toFormValue(
+        string $key,
+        string|bool|array|DateTime|ArrayAccess|\SplFileObject $value,
+    ): array {
         if ($value instanceof \SplFileObject) {
-            return $value->getRealPath();
+            return [$key => $value->getRealPath()];
+        } elseif (is_array($value) || $value instanceof ArrayAccess) {
+            $flattened = [];
+            $result = [];
+
+            self::flattenArray(json_decode(json_encode($value), true), $flattened);
+
+            foreach ($flattened as $k => $v) {
+                $result["{$key}{$k}"] = self::toString($v);
+            }
+
+            return $result;
         } else {
-            return self::toString($value);
+            return [$key => self::toString($value)];
         }
     }
 
@@ -607,4 +621,58 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * credit: https://github.com/FranBar1966/FlatPHP
+     */
+    private static function flattenArray(
+        ArrayAccess|array $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flattenArray($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }
diff --git a/samples/client/petstore/php/OpenAPIClient-php/README.md b/samples/client/petstore/php/OpenAPIClient-php/README.md
index 18918e09409..d6f67751a75 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/README.md
+++ b/samples/client/petstore/php/OpenAPIClient-php/README.md
@@ -107,6 +107,7 @@ Class | Method | HTTP request | Description
 *PetApi* | [**updatePetWithForm**](docs/Api/PetApi.md#updatepetwithform) | **POST** /pet/{petId} | Updates a pet in the store with form data
 *PetApi* | [**uploadFile**](docs/Api/PetApi.md#uploadfile) | **POST** /pet/{petId}/uploadImage | uploads an image
 *PetApi* | [**uploadFileWithRequiredFile**](docs/Api/PetApi.md#uploadfilewithrequiredfile) | **POST** /fake/{petId}/uploadImageWithRequiredFile | uploads an image (required)
+*PetApi* | [**uploadImageFullFormData**](docs/Api/PetApi.md#uploadimagefullformdata) | **POST** /pet/{petId}/uploadImageFullFormData | uploads an image attached to a Pet object as formdata
 *StoreApi* | [**deleteOrder**](docs/Api/StoreApi.md#deleteorder) | **DELETE** /store/order/{order_id} | Delete purchase order by ID
 *StoreApi* | [**getInventory**](docs/Api/StoreApi.md#getinventory) | **GET** /store/inventory | Returns pet inventories by status
 *StoreApi* | [**getOrderById**](docs/Api/StoreApi.md#getorderbyid) | **GET** /store/order/{order_id} | Find purchase order by ID
diff --git a/samples/client/petstore/php/OpenAPIClient-php/docs/Api/PetApi.md b/samples/client/petstore/php/OpenAPIClient-php/docs/Api/PetApi.md
index 3a832aa82a5..b81cba25df6 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/docs/Api/PetApi.md
+++ b/samples/client/petstore/php/OpenAPIClient-php/docs/Api/PetApi.md
@@ -13,6 +13,7 @@ All URIs are relative to http://petstore.swagger.io:80/v2, except if the operati
 | [**updatePetWithForm()**](PetApi.md#updatePetWithForm) | **POST** /pet/{petId} | Updates a pet in the store with form data |
 | [**uploadFile()**](PetApi.md#uploadFile) | **POST** /pet/{petId}/uploadImage | uploads an image |
 | [**uploadFileWithRequiredFile()**](PetApi.md#uploadFileWithRequiredFile) | **POST** /fake/{petId}/uploadImageWithRequiredFile | uploads an image (required) |
+| [**uploadImageFullFormData()**](PetApi.md#uploadImageFullFormData) | **POST** /pet/{petId}/uploadImageFullFormData | uploads an image attached to a Pet object as formdata |
 
 
 ## `addPet()`
@@ -612,3 +613,79 @@ try {
 [[Back to top]](#) [[Back to API list]](../../README.md#endpoints)
 [[Back to Model list]](../../README.md#models)
 [[Back to README]](../../README.md)
+
+## `uploadImageFullFormData()`
+
+```php
+uploadImageFullFormData($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files): \OpenAPI\Client\Model\ApiResponse
+```
+
+uploads an image attached to a Pet object as formdata
+
+
+
+### Example
+
+```php
+<?php
+require_once(__DIR__ . '/vendor/autoload.php');
+
+
+// Configure OAuth2 access token for authorization: petstore_auth
+$config = OpenAPI\Client\Configuration::getDefaultConfiguration()->setAccessToken('YOUR_ACCESS_TOKEN');
+
+
+$apiInstance = new OpenAPI\Client\Api\PetApi(
+    // If you want use custom http client, pass your client which implements `GuzzleHttp\ClientInterface`.
+    // This is optional, `GuzzleHttp\Client` will be used as default.
+    new GuzzleHttp\Client(),
+    $config
+);
+$pet_id = 56; // int | ID of pet to update
+$name = 'name_example'; // string
+$photo_urls = array('photo_urls_example'); // string[]
+$id = 56; // int
+$category = new \OpenAPI\Client\Model\Category(); // \OpenAPI\Client\Model\Category
+$tags = array(new \OpenAPI\Client\Model\\OpenAPI\Client\Model\Tag()); // \OpenAPI\Client\Model\Tag[]
+$status = 'status_example'; // string | pet status in the store
+$file = '/path/to/file.txt'; // \SplFileObject | file to upload
+$multiple_files = array('/path/to/file.txt'); // \SplFileObject[]
+
+try {
+    $result = $apiInstance->uploadImageFullFormData($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files);
+    print_r($result);
+} catch (Exception $e) {
+    echo 'Exception when calling PetApi->uploadImageFullFormData: ', $e->getMessage(), PHP_EOL;
+}
+```
+
+### Parameters
+
+| Name | Type | Description  | Notes |
+| ------------- | ------------- | ------------- | ------------- |
+| **pet_id** | **int**| ID of pet to update | |
+| **name** | **string**|  | |
+| **photo_urls** | [**string[]**](../Model/string.md)|  | |
+| **id** | **int**|  | [optional] |
+| **category** | [**\OpenAPI\Client\Model\Category**](../Model/Category.md)|  | [optional] |
+| **tags** | [**\OpenAPI\Client\Model\Tag[]**](../Model/\OpenAPI\Client\Model\Tag.md)|  | [optional] |
+| **status** | **string**| pet status in the store | [optional] |
+| **file** | **\SplFileObject****\SplFileObject**| file to upload | [optional] |
+| **multiple_files** | **\SplFileObject[]**|  | [optional] |
+
+### Return type
+
+[**\OpenAPI\Client\Model\ApiResponse**](../Model/ApiResponse.md)
+
+### Authorization
+
+[petstore_auth](../../README.md#petstore_auth)
+
+### HTTP request headers
+
+- **Content-Type**: `multipart/form-data`
+- **Accept**: `application/json`
+
+[[Back to top]](#) [[Back to API list]](../../README.md#endpoints)
+[[Back to Model list]](../../README.md#models)
+[[Back to README]](../../README.md)
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 8c2cde794df..a89d506fec6 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
@@ -4567,39 +4567,39 @@ class FakeApi
 
         // form params
         if ($integer !== null) {
-            $formParams['integer'] = ObjectSerializer::toFormValue($integer);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('integer', $integer));
         }
         // form params
         if ($int32 !== null) {
-            $formParams['int32'] = ObjectSerializer::toFormValue($int32);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('int32', $int32));
         }
         // form params
         if ($int64 !== null) {
-            $formParams['int64'] = ObjectSerializer::toFormValue($int64);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('int64', $int64));
         }
         // form params
         if ($number !== null) {
-            $formParams['number'] = ObjectSerializer::toFormValue($number);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('number', $number));
         }
         // form params
         if ($float !== null) {
-            $formParams['float'] = ObjectSerializer::toFormValue($float);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('float', $float));
         }
         // form params
         if ($double !== null) {
-            $formParams['double'] = ObjectSerializer::toFormValue($double);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('double', $double));
         }
         // form params
         if ($string !== null) {
-            $formParams['string'] = ObjectSerializer::toFormValue($string);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('string', $string));
         }
         // form params
         if ($pattern_without_delimiter !== null) {
-            $formParams['pattern_without_delimiter'] = ObjectSerializer::toFormValue($pattern_without_delimiter);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('pattern_without_delimiter', $pattern_without_delimiter));
         }
         // form params
         if ($byte !== null) {
-            $formParams['byte'] = ObjectSerializer::toFormValue($byte);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('byte', $byte));
         }
         // form params
         if ($binary !== null) {
@@ -4608,26 +4608,26 @@ class FakeApi
             $paramFiles = is_array($binary) ? $binary : [$binary];
             foreach ($paramFiles as $paramFile) {
                 $formParams['binary'][] = \GuzzleHttp\Psr7\Utils::tryFopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('binary', $paramFile)['binary'],
                     'rb'
                 );
             }
         }
         // form params
         if ($date !== null) {
-            $formParams['date'] = ObjectSerializer::toFormValue($date);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('date', $date));
         }
         // form params
         if ($date_time !== null) {
-            $formParams['dateTime'] = ObjectSerializer::toFormValue($date_time);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('dateTime', $date_time));
         }
         // form params
         if ($password !== null) {
-            $formParams['password'] = ObjectSerializer::toFormValue($password);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('password', $password));
         }
         // form params
         if ($callback !== null) {
-            $formParams['callback'] = ObjectSerializer::toFormValue($callback);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('callback', $callback));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -4942,11 +4942,11 @@ class FakeApi
 
         // form params
         if ($enum_form_string_array !== null) {
-            $formParams['enum_form_string_array'] = ObjectSerializer::toFormValue($enum_form_string_array);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('enum_form_string_array', $enum_form_string_array));
         }
         // form params
         if ($enum_form_string !== null) {
-            $formParams['enum_form_string'] = ObjectSerializer::toFormValue($enum_form_string);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('enum_form_string', $enum_form_string));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -5920,11 +5920,11 @@ class FakeApi
 
         // form params
         if ($param !== null) {
-            $formParams['param'] = ObjectSerializer::toFormValue($param);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('param', $param));
         }
         // form params
         if ($param2 !== null) {
-            $formParams['param2'] = ObjectSerializer::toFormValue($param2);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('param2', $param2));
         }
 
         $headers = $this->headerSelector->selectHeaders(
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 9313bf4f635..afc732bd153 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
@@ -100,6 +100,9 @@ class PetApi
         'uploadFileWithRequiredFile' => [
             'multipart/form-data',
         ],
+        'uploadImageFullFormData' => [
+            'multipart/form-data',
+        ],
     ];
 
     /**
@@ -2224,11 +2227,11 @@ class PetApi
 
         // form params
         if ($name !== null) {
-            $formParams['name'] = ObjectSerializer::toFormValue($name);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('name', $name));
         }
         // form params
         if ($status !== null) {
-            $formParams['status'] = ObjectSerializer::toFormValue($status);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('status', $status));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -2556,7 +2559,7 @@ class PetApi
 
         // form params
         if ($additional_metadata !== null) {
-            $formParams['additionalMetadata'] = ObjectSerializer::toFormValue($additional_metadata);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('additionalMetadata', $additional_metadata));
         }
         // form params
         if ($file !== null) {
@@ -2565,7 +2568,7 @@ class PetApi
             $paramFiles = is_array($file) ? $file : [$file];
             foreach ($paramFiles as $paramFile) {
                 $formParams['file'][] = \GuzzleHttp\Psr7\Utils::tryFopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('file', $paramFile)['file'],
                     'rb'
                 );
             }
@@ -2903,7 +2906,7 @@ class PetApi
 
         // form params
         if ($additional_metadata !== null) {
-            $formParams['additionalMetadata'] = ObjectSerializer::toFormValue($additional_metadata);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('additionalMetadata', $additional_metadata));
         }
         // form params
         if ($required_file !== null) {
@@ -2912,7 +2915,428 @@ class PetApi
             $paramFiles = is_array($required_file) ? $required_file : [$required_file];
             foreach ($paramFiles as $paramFile) {
                 $formParams['requiredFile'][] = \GuzzleHttp\Psr7\Utils::tryFopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('requiredFile', $paramFile)['requiredFile'],
+                    'rb'
+                );
+            }
+        }
+
+        $multipart = true;
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json', ],
+            $contentType,
+            $multipart
+        );
+
+        // for model (json/xml)
+        if (count($formParams) > 0) {
+            if ($multipart) {
+                $multipartContents = [];
+                foreach ($formParams as $formParamName => $formParamValue) {
+                    $formParamValueItems = is_array($formParamValue) ? $formParamValue : [$formParamValue];
+                    foreach ($formParamValueItems as $formParamValueItem) {
+                        $multipartContents[] = [
+                            'name' => $formParamName,
+                            'contents' => $formParamValueItem
+                        ];
+                    }
+                }
+                // for HTTP post (form)
+                $httpBody = new MultipartStream($multipartContents);
+
+            } elseif (stripos($headers['Content-Type'], 'application/json') !== false) {
+                # if Content-Type contains "application/json", json_encode the form parameters
+                $httpBody = \GuzzleHttp\Utils::jsonEncode($formParams);
+            } else {
+                // for HTTP post (form)
+                $httpBody = ObjectSerializer::buildQuery($formParams);
+            }
+        }
+
+        // this endpoint requires OAuth (access token)
+        if (!empty($this->config->getAccessToken())) {
+            $headers['Authorization'] = 'Bearer ' . $this->config->getAccessToken();
+        }
+
+        $defaultHeaders = [];
+        if ($this->config->getUserAgent()) {
+            $defaultHeaders['User-Agent'] = $this->config->getUserAgent();
+        }
+
+        $headers = array_merge(
+            $defaultHeaders,
+            $headerParams,
+            $headers
+        );
+
+        $operationHost = $this->config->getHost();
+        $query = ObjectSerializer::buildQuery($queryParams);
+        return new Request(
+            'POST',
+            $operationHost . $resourcePath . ($query ? "?{$query}" : ''),
+            $headers,
+            $httpBody
+        );
+    }
+
+    /**
+     * Operation uploadImageFullFormData
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name name (required)
+     * @param  string[] $photo_urls photo_urls (required)
+     * @param  int|null $id id (optional)
+     * @param  \OpenAPI\Client\Model\Category|null $category category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[]|null $tags tags (optional)
+     * @param  string|null $status pet status in the store (optional)
+     * @param  \SplFileObject|null $file file to upload (optional)
+     * @param  \SplFileObject[]|null $multiple_files multiple_files (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadImageFullFormData'] to see the possible values for this operation
+     *
+     * @throws \OpenAPI\Client\ApiException on non-2xx response or if the response body is not in the expected format
+     * @throws \InvalidArgumentException
+     * @return \OpenAPI\Client\Model\ApiResponse
+     */
+    public function uploadImageFullFormData($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null, string $contentType = self::contentTypes['uploadImageFullFormData'][0])
+    {
+        list($response) = $this->uploadImageFullFormDataWithHttpInfo($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files, $contentType);
+        return $response;
+    }
+
+    /**
+     * Operation uploadImageFullFormDataWithHttpInfo
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int|null $id (optional)
+     * @param  \OpenAPI\Client\Model\Category|null $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[]|null $tags (optional)
+     * @param  string|null $status pet status in the store (optional)
+     * @param  \SplFileObject|null $file file to upload (optional)
+     * @param  \SplFileObject[]|null $multiple_files (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadImageFullFormData'] to see the possible values for this operation
+     *
+     * @throws \OpenAPI\Client\ApiException on non-2xx response or if the response body is not in the expected format
+     * @throws \InvalidArgumentException
+     * @return array of \OpenAPI\Client\Model\ApiResponse, HTTP status code, HTTP response headers (array of strings)
+     */
+    public function uploadImageFullFormDataWithHttpInfo($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null, string $contentType = self::contentTypes['uploadImageFullFormData'][0])
+    {
+        $request = $this->uploadImageFullFormDataRequest($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files, $contentType);
+
+        try {
+            $options = $this->createHttpClientOption();
+            try {
+                $response = $this->client->send($request, $options);
+            } catch (RequestException $e) {
+                throw new ApiException(
+                    "[{$e->getCode()}] {$e->getMessage()}",
+                    (int) $e->getCode(),
+                    $e->getResponse() ? $e->getResponse()->getHeaders() : null,
+                    $e->getResponse() ? (string) $e->getResponse()->getBody() : null
+                );
+            } catch (ConnectException $e) {
+                throw new ApiException(
+                    "[{$e->getCode()}] {$e->getMessage()}",
+                    (int) $e->getCode(),
+                    null,
+                    null
+                );
+            }
+
+            $statusCode = $response->getStatusCode();
+
+
+            switch($statusCode) {
+                case 200:
+                    if ('\OpenAPI\Client\Model\ApiResponse' === '\SplFileObject') {
+                        $content = $response->getBody(); //stream goes to serializer
+                    } else {
+                        $content = (string) $response->getBody();
+                        if ('\OpenAPI\Client\Model\ApiResponse' !== 'string') {
+                            try {
+                                $content = json_decode($content, false, 512, JSON_THROW_ON_ERROR);
+                            } catch (\JsonException $exception) {
+                                throw new ApiException(
+                                    sprintf(
+                                        'Error JSON decoding server response (%s)',
+                                        $request->getUri()
+                                    ),
+                                    $statusCode,
+                                    $response->getHeaders(),
+                                    $content
+                                );
+                            }
+                        }
+                    }
+
+                    return [
+                        ObjectSerializer::deserialize($content, '\OpenAPI\Client\Model\ApiResponse', []),
+                        $response->getStatusCode(),
+                        $response->getHeaders()
+                    ];
+            }
+
+            if ($statusCode < 200 || $statusCode > 299) {
+                throw new ApiException(
+                    sprintf(
+                        '[%d] Error connecting to the API (%s)',
+                        $statusCode,
+                        (string) $request->getUri()
+                    ),
+                    $statusCode,
+                    $response->getHeaders(),
+                    (string) $response->getBody()
+                );
+            }
+
+            $returnType = '\OpenAPI\Client\Model\ApiResponse';
+            if ($returnType === '\SplFileObject') {
+                $content = $response->getBody(); //stream goes to serializer
+            } else {
+                $content = (string) $response->getBody();
+                if ($returnType !== 'string') {
+                    try {
+                        $content = json_decode($content, false, 512, JSON_THROW_ON_ERROR);
+                    } catch (\JsonException $exception) {
+                        throw new ApiException(
+                            sprintf(
+                                'Error JSON decoding server response (%s)',
+                                $request->getUri()
+                            ),
+                            $statusCode,
+                            $response->getHeaders(),
+                            $content
+                        );
+                    }
+                }
+            }
+
+            return [
+                ObjectSerializer::deserialize($content, $returnType, []),
+                $response->getStatusCode(),
+                $response->getHeaders()
+            ];
+
+        } catch (ApiException $e) {
+            switch ($e->getCode()) {
+                case 200:
+                    $data = ObjectSerializer::deserialize(
+                        $e->getResponseBody(),
+                        '\OpenAPI\Client\Model\ApiResponse',
+                        $e->getResponseHeaders()
+                    );
+                    $e->setResponseObject($data);
+                    break;
+            }
+            throw $e;
+        }
+    }
+
+    /**
+     * Operation uploadImageFullFormDataAsync
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int|null $id (optional)
+     * @param  \OpenAPI\Client\Model\Category|null $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[]|null $tags (optional)
+     * @param  string|null $status pet status in the store (optional)
+     * @param  \SplFileObject|null $file file to upload (optional)
+     * @param  \SplFileObject[]|null $multiple_files (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadImageFullFormData'] to see the possible values for this operation
+     *
+     * @throws \InvalidArgumentException
+     * @return \GuzzleHttp\Promise\PromiseInterface
+     */
+    public function uploadImageFullFormDataAsync($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null, string $contentType = self::contentTypes['uploadImageFullFormData'][0])
+    {
+        return $this->uploadImageFullFormDataAsyncWithHttpInfo($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files, $contentType)
+            ->then(
+                function ($response) {
+                    return $response[0];
+                }
+            );
+    }
+
+    /**
+     * Operation uploadImageFullFormDataAsyncWithHttpInfo
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int|null $id (optional)
+     * @param  \OpenAPI\Client\Model\Category|null $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[]|null $tags (optional)
+     * @param  string|null $status pet status in the store (optional)
+     * @param  \SplFileObject|null $file file to upload (optional)
+     * @param  \SplFileObject[]|null $multiple_files (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadImageFullFormData'] to see the possible values for this operation
+     *
+     * @throws \InvalidArgumentException
+     * @return \GuzzleHttp\Promise\PromiseInterface
+     */
+    public function uploadImageFullFormDataAsyncWithHttpInfo($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null, string $contentType = self::contentTypes['uploadImageFullFormData'][0])
+    {
+        $returnType = '\OpenAPI\Client\Model\ApiResponse';
+        $request = $this->uploadImageFullFormDataRequest($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files, $contentType);
+
+        return $this->client
+            ->sendAsync($request, $this->createHttpClientOption())
+            ->then(
+                function ($response) use ($returnType) {
+                    if ($returnType === '\SplFileObject') {
+                        $content = $response->getBody(); //stream goes to serializer
+                    } else {
+                        $content = (string) $response->getBody();
+                        if ($returnType !== 'string') {
+                            $content = json_decode($content);
+                        }
+                    }
+
+                    return [
+                        ObjectSerializer::deserialize($content, $returnType, []),
+                        $response->getStatusCode(),
+                        $response->getHeaders()
+                    ];
+                },
+                function ($exception) {
+                    $response = $exception->getResponse();
+                    $statusCode = $response->getStatusCode();
+                    throw new ApiException(
+                        sprintf(
+                            '[%d] Error connecting to the API (%s)',
+                            $statusCode,
+                            $exception->getRequest()->getUri()
+                        ),
+                        $statusCode,
+                        $response->getHeaders(),
+                        (string) $response->getBody()
+                    );
+                }
+            );
+    }
+
+    /**
+     * Create request for operation 'uploadImageFullFormData'
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int|null $id (optional)
+     * @param  \OpenAPI\Client\Model\Category|null $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[]|null $tags (optional)
+     * @param  string|null $status pet status in the store (optional)
+     * @param  \SplFileObject|null $file file to upload (optional)
+     * @param  \SplFileObject[]|null $multiple_files (optional)
+     * @param  string $contentType The value for the Content-Type header. Check self::contentTypes['uploadImageFullFormData'] to see the possible values for this operation
+     *
+     * @throws \InvalidArgumentException
+     * @return \GuzzleHttp\Psr7\Request
+     */
+    public function uploadImageFullFormDataRequest($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null, string $contentType = self::contentTypes['uploadImageFullFormData'][0])
+    {
+
+        // verify the required parameter 'pet_id' is set
+        if ($pet_id === null || (is_array($pet_id) && count($pet_id) === 0)) {
+            throw new \InvalidArgumentException(
+                'Missing the required parameter $pet_id when calling uploadImageFullFormData'
+            );
+        }
+
+        // verify the required parameter 'name' is set
+        if ($name === null || (is_array($name) && count($name) === 0)) {
+            throw new \InvalidArgumentException(
+                'Missing the required parameter $name when calling uploadImageFullFormData'
+            );
+        }
+
+        // verify the required parameter 'photo_urls' is set
+        if ($photo_urls === null || (is_array($photo_urls) && count($photo_urls) === 0)) {
+            throw new \InvalidArgumentException(
+                'Missing the required parameter $photo_urls when calling uploadImageFullFormData'
+            );
+        }
+        
+
+
+
+
+
+
+
+        $resourcePath = '/pet/{petId}/uploadImageFullFormData';
+        $formParams = [];
+        $queryParams = [];
+        $headerParams = [];
+        $httpBody = '';
+        $multipart = false;
+
+
+
+        // path params
+        if ($pet_id !== null) {
+            $resourcePath = str_replace(
+                '{' . 'petId' . '}',
+                ObjectSerializer::toPathValue($pet_id),
+                $resourcePath
+            );
+        }
+
+        // form params
+        if ($id !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('id', $id));
+        }
+        // form params
+        if ($category !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('category', $category));
+        }
+        // form params
+        if ($name !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('name', $name));
+        }
+        // form params
+        if ($photo_urls !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('photoUrls', $photo_urls));
+        }
+        // form params
+        if ($tags !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('tags', $tags));
+        }
+        // form params
+        if ($status !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('status', $status));
+        }
+        // form params
+        if ($file !== null) {
+            $multipart = true;
+            $formParams['file'] = [];
+            $paramFiles = is_array($file) ? $file : [$file];
+            foreach ($paramFiles as $paramFile) {
+                $formParams['file'][] = \GuzzleHttp\Psr7\Utils::tryFopen(
+                    ObjectSerializer::toFormValue('file', $paramFile)['file'],
+                    'rb'
+                );
+            }
+        }
+        // form params
+        if ($multiple_files !== null) {
+            $multipart = true;
+            $formParams['multiple_files'] = [];
+            $paramFiles = is_array($multiple_files) ? $multiple_files : [$multiple_files];
+            foreach ($paramFiles as $paramFile) {
+                $formParams['multiple_files'][] = \GuzzleHttp\Psr7\Utils::tryFopen(
+                    ObjectSerializer::toFormValue('multiple_files', $paramFile)['multiple_files'],
                     'rb'
                 );
             }
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php b/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
index 440a243a41a..3e1cbe8778b 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
@@ -28,6 +28,7 @@
 
 namespace OpenAPI\Client;
 
+use ArrayAccess;
 use GuzzleHttp\Psr7\Utils;
 use OpenAPI\Client\Model\ModelInterface;
 
@@ -324,20 +325,31 @@ class ObjectSerializer
     }
 
     /**
-     * Take value and turn it into a string suitable for inclusion in
+     * Take value and turn it into an array suitable for inclusion in
      * the http body (form parameter). If it's a string, pass through unchanged
      * If it's a datetime object, format it in ISO8601
      *
-     * @param string|\SplFileObject $value the value of the form parameter
+     * @param string|bool|array|DateTime|ArrayAccess|\SplFileObject $value the value of the form parameter
      *
-     * @return string the form string
+     * @return array [key => value] of formdata
      */
-    public static function toFormValue($value)
+    public static function toFormValue(string $key, mixed $value)
     {
         if ($value instanceof \SplFileObject) {
-            return $value->getRealPath();
+            return [$key => $value->getRealPath()];
+        } elseif (is_array($value) || $value instanceof ArrayAccess) {
+            $flattened = [];
+            $result = [];
+
+            self::flattenArray(json_decode(json_encode($value), true), $flattened);
+
+            foreach ($flattened as $k => $v) {
+                $result["{$key}{$k}"] = self::toString($v);
+            }
+
+            return $result;
         } else {
-            return self::toString($value);
+            return [$key => self::toString($value)];
         }
     }
 
@@ -614,4 +626,81 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * @param \ArrayAccess|array $source
+     *
+     * credit: https://github.com/FranBar1966/FlatPHP
+     */
+    private static function flattenArray(
+        mixed $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        /**
+         * array_is_list only in PHP >= 8.1
+         *
+         * credit: https://www.php.net/manual/en/function.array-is-list.php#127044
+         */
+        if (!function_exists('array_is_list')) {
+            function array_is_list(array $array)
+            {
+                $i = -1;
+
+                foreach ($array as $k => $v) {
+                    ++$i;
+                    if ($k !== $i) {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flattenArray($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }
diff --git a/samples/client/petstore/php/OpenAPIClient-php/tests/ObjectSerializerTest.php b/samples/client/petstore/php/OpenAPIClient-php/tests/ObjectSerializerTest.php
index dc71cd108b8..47b94f12f4f 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/tests/ObjectSerializerTest.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/tests/ObjectSerializerTest.php
@@ -636,4 +636,98 @@ class ObjectSerializerTest extends TestCase
         $tag = $tags[0];
         $this->assertInstanceOf(Tag::class, $tag);
     }
+
+    /**
+     * @dataProvider providerToFormValue
+     */
+    public function testToFormValue(
+        mixed $data,
+        mixed $expected,
+    ): void {
+        $result = ObjectSerializer::toFormValue('key', $data);
+
+        $this->assertEquals($expected, $result);
+    }
+
+    public function providerToFormValue(): iterable
+    {
+        yield [
+            'data'     => new DateTime('2021-10-06T20:17:16'),
+            'expected' => ['key' => '2021-10-06T20:17:16+00:00'],
+        ];
+
+        yield [
+            'data'     => true,
+            'expected' => ['key' => 'true'],
+        ];
+
+        yield [
+            'data'     => false,
+            'expected' => ['key' => 'false'],
+        ];
+
+        yield [
+            'data'     => 'some value',
+            'expected' => ['key' => 'some value'],
+        ];
+
+        $filepath = realpath(__DIR__ . '/../.openapi-generator/VERSION');
+        $file = new \SplFileObject($filepath);
+
+        yield [
+            'data'     => $file,
+            'expected' => ['key' => $filepath],
+        ];
+
+        $id = 1234;
+        $name = 'Spike';
+
+        $category = (new Model\Category())
+            ->setId(12345)
+            ->setName("Category_Name");
+
+        $tags_1 = (new Model\Tag())
+            ->setId(12345)
+            ->setName("tag_1");
+
+        $tags_2 = (new Model\Tag())
+            ->setId(98765)
+            ->setName("tag_2");
+
+        $tags = [
+            $tags_1,
+            $tags_2,
+        ];
+
+        $photo_urls = [
+            "https://example.com/picture_1.jpg",
+            "https://example.com/picture_2.jpg",
+        ];
+        $status = Model\Pet::STATUS_AVAILABLE;
+
+        $pet = new Model\Pet([]);
+        $pet->setId($id)
+            ->setName($name)
+            ->setPhotoUrls($photo_urls)
+            ->setStatus($status)
+            ->setCategory($category)
+            ->setTags($tags);
+
+        yield [
+            'data'     => $pet,
+            'expected' => [
+                'key[id]'             => "{$id}",
+                'key[name]'           => $name,
+                'key[photoUrls][0]'   => $photo_urls[0],
+                'key[photoUrls][1]'   => $photo_urls[1],
+                'key[status]'         => $status,
+                'key[category][id]'   => "{$category->getId()}",
+                'key[category][name]' => $category->getName(),
+                'key[tags][0][id]'    => "{$tags_1->getId()}",
+                'key[tags][0][name]'  => $tags_1->getName(),
+                'key[tags][1][id]'    => "{$tags_2->getId()}",
+                'key[tags][1][name]'  => $tags_2->getName(),
+            ],
+        ];
+    }
 }
diff --git a/samples/client/petstore/php/OpenAPIClient-php/tests/PetApiTest.php b/samples/client/petstore/php/OpenAPIClient-php/tests/PetApiTest.php
index e7a8b97419a..463f36a78af 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/tests/PetApiTest.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/tests/PetApiTest.php
@@ -45,9 +45,6 @@ class PetApiTest extends TestCase
         $config = new Configuration();
         $petApi = new Api\PetApi(null, $config);
 
-        // add a new pet (model)
-        list(, $status) = $petApi->addPetWithHttpInfo($newPet);
-        Assert::assertEquals(200, $status);
     }
 
     public function setUp(): void
@@ -371,6 +368,84 @@ class PetApiTest extends TestCase
         $this->api->findPetsByStatus([]);
     }
 
+    public function testObjectInFormData()
+    {
+        $category = (new Model\Category())
+            ->setId(12345)
+            ->setName("Category_Name");
+
+        $tags_1 = (new Model\Tag())
+            ->setId(12345)
+            ->setName("tag_1");
+
+        $tags_2 = (new Model\Tag())
+            ->setId(98765)
+            ->setName("tag_2");
+
+        /** @var Model\Tag[] $tags */
+        $tags = [
+            $tags_1,
+            $tags_2,
+        ];
+
+        $pet_id = 56;
+        $name = "My pet name";
+        $photo_urls = [
+            "https://example.com/picture_1.jpg",
+            "https://example.com/picture_2.jpg",
+        ];
+        $id = 12345;
+        $status = Model\Pet::STATUS_AVAILABLE;
+        $file = new \SplFileObject(__DIR__ . '/../.openapi-generator/VERSION');
+        $multiple_files = [
+            $file,
+            $file,
+        ];
+
+        $request = (new Api\PetApi())->uploadImageFullFormDataRequest(
+            $pet_id,
+            $name,
+            $photo_urls,
+            $id,
+            $category,
+            $tags,
+            $status,
+            $file,
+            $multiple_files,
+        );
+
+        $contents = $request->getBody()->getContents();
+
+        $this->assertBodyContents('name', $name, $contents);
+        $this->assertBodyContents('photoUrls[0]', $photo_urls[0], $contents);
+        $this->assertBodyContents('photoUrls[1]', $photo_urls[1], $contents);
+        $this->assertBodyContents('category[id]', $category->getId(), $contents);
+        $this->assertBodyContents('category[name]', $category->getName(), $contents);
+        $this->assertBodyContents('tags[0][id]', $tags[0]->getId(), $contents);
+        $this->assertBodyContents('tags[0][name]', $tags[0]->getName(), $contents);
+        $this->assertBodyContents('tags[1][id]', $tags[1]->getId(), $contents);
+        $this->assertBodyContents('tags[1][name]', $tags[1]->getName(), $contents);
+        $this->assertBodyContents('status', $status, $contents);
+    }
+
+    private function assertBodyContents(
+        string $name,
+        mixed $value,
+        string $contents,
+    ) {
+        $length = strlen((string) ($value));
+        $contents = implode("\n", array_map('trim', explode("\n", $contents)));
+
+        $expected = <<<END
+            Content-Disposition: form-data; name="{$name}"
+            Content-Length: {$length}
+            
+            {$value}
+            END;
+
+        $this->assertStringContainsString($expected, $contents);
+    }
+
 //    Disabled as currently we don't have any endpoint that would return file
 //    For testing I just replaced url and return type in Api method.
 //    public function testDownloadingLargeFile()
diff --git a/samples/client/petstore/php/psr-18/README.md b/samples/client/petstore/php/psr-18/README.md
index 0dde61cbcb7..1c8d4f2541c 100644
--- a/samples/client/petstore/php/psr-18/README.md
+++ b/samples/client/petstore/php/psr-18/README.md
@@ -118,6 +118,7 @@ Class | Method | HTTP request | Description
 *PetApi* | [**updatePetWithForm**](docs/Api/PetApi.md#updatepetwithform) | **POST** /pet/{petId} | Updates a pet in the store with form data
 *PetApi* | [**uploadFile**](docs/Api/PetApi.md#uploadfile) | **POST** /pet/{petId}/uploadImage | uploads an image
 *PetApi* | [**uploadFileWithRequiredFile**](docs/Api/PetApi.md#uploadfilewithrequiredfile) | **POST** /fake/{petId}/uploadImageWithRequiredFile | uploads an image (required)
+*PetApi* | [**uploadImageFullFormData**](docs/Api/PetApi.md#uploadimagefullformdata) | **POST** /pet/{petId}/uploadImageFullFormData | uploads an image attached to a Pet object as formdata
 *StoreApi* | [**deleteOrder**](docs/Api/StoreApi.md#deleteorder) | **DELETE** /store/order/{order_id} | Delete purchase order by ID
 *StoreApi* | [**getInventory**](docs/Api/StoreApi.md#getinventory) | **GET** /store/inventory | Returns pet inventories by status
 *StoreApi* | [**getOrderById**](docs/Api/StoreApi.md#getorderbyid) | **GET** /store/order/{order_id} | Find purchase order by ID
diff --git a/samples/client/petstore/php/psr-18/docs/Api/PetApi.md b/samples/client/petstore/php/psr-18/docs/Api/PetApi.md
index 1c7add7728a..1b5a4c942fd 100644
--- a/samples/client/petstore/php/psr-18/docs/Api/PetApi.md
+++ b/samples/client/petstore/php/psr-18/docs/Api/PetApi.md
@@ -13,6 +13,7 @@ Method | HTTP request | Description
 [**updatePetWithForm()**](PetApi.md#updatePetWithForm) | **POST** /pet/{petId} | Updates a pet in the store with form data
 [**uploadFile()**](PetApi.md#uploadFile) | **POST** /pet/{petId}/uploadImage | uploads an image
 [**uploadFileWithRequiredFile()**](PetApi.md#uploadFileWithRequiredFile) | **POST** /fake/{petId}/uploadImageWithRequiredFile | uploads an image (required)
+[**uploadImageFullFormData()**](PetApi.md#uploadImageFullFormData) | **POST** /pet/{petId}/uploadImageFullFormData | uploads an image attached to a Pet object as formdata
 
 
 ## `addPet()`
@@ -566,3 +567,79 @@ Name | Type | Description  | Notes
 [[Back to top]](#) [[Back to API list]](../../README.md#endpoints)
 [[Back to Model list]](../../README.md#models)
 [[Back to README]](../../README.md)
+
+## `uploadImageFullFormData()`
+
+```php
+uploadImageFullFormData($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files): \OpenAPI\Client\Model\ApiResponse
+```
+
+uploads an image attached to a Pet object as formdata
+
+
+
+### Example
+
+```php
+<?php
+require_once(__DIR__ . '/vendor/autoload.php');
+
+
+// Configure OAuth2 access token for authorization: petstore_auth
+$config = OpenAPI\Client\Configuration::getDefaultConfiguration()->setAccessToken('YOUR_ACCESS_TOKEN');
+
+
+$apiInstance = new OpenAPI\Client\Api\PetApi(
+    // If you want use custom http client, pass your client which implements `Psr\Http\Client\ClientInterface`.
+    // This is optional, `Psr18ClientDiscovery` will be used to find http client. For instance `GuzzleHttp\Client` implements that interface
+    new GuzzleHttp\Client(),
+    $config
+);
+$pet_id = 56; // int | ID of pet to update
+$name = 'name_example'; // string
+$photo_urls = array('photo_urls_example'); // string[]
+$id = 56; // int
+$category = new \OpenAPI\Client\Model\Category(); // \OpenAPI\Client\Model\Category
+$tags = array(new \OpenAPI\Client\Model\\OpenAPI\Client\Model\Tag()); // \OpenAPI\Client\Model\Tag[]
+$status = 'status_example'; // string | pet status in the store
+$file = '/path/to/file.txt'; // \SplFileObject | file to upload
+$multiple_files = array('/path/to/file.txt'); // \SplFileObject[]
+
+try {
+    $result = $apiInstance->uploadImageFullFormData($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files);
+    print_r($result);
+} catch (Exception $e) {
+    echo 'Exception when calling PetApi->uploadImageFullFormData: ', $e->getMessage(), PHP_EOL;
+}
+```
+
+### Parameters
+
+Name | Type | Description  | Notes
+------------- | ------------- | ------------- | -------------
+ **pet_id** | **int**| ID of pet to update |
+ **name** | **string**|  |
+ **photo_urls** | [**string[]**](../Model/string.md)|  |
+ **id** | **int**|  | [optional]
+ **category** | [**\OpenAPI\Client\Model\Category**](../Model/Category.md)|  | [optional]
+ **tags** | [**\OpenAPI\Client\Model\Tag[]**](../Model/\OpenAPI\Client\Model\Tag.md)|  | [optional]
+ **status** | **string**| pet status in the store | [optional]
+ **file** | **\SplFileObject****\SplFileObject**| file to upload | [optional]
+ **multiple_files** | **\SplFileObject[]**|  | [optional]
+
+### Return type
+
+[**\OpenAPI\Client\Model\ApiResponse**](../Model/ApiResponse.md)
+
+### Authorization
+
+[petstore_auth](../../README.md#petstore_auth)
+
+### HTTP request headers
+
+- **Content-Type**: `multipart/form-data`
+- **Accept**: `application/json`
+
+[[Back to top]](#) [[Back to API list]](../../README.md#endpoints)
+[[Back to Model list]](../../README.md#models)
+[[Back to README]](../../README.md)
diff --git a/samples/client/petstore/php/psr-18/lib/Api/FakeApi.php b/samples/client/petstore/php/psr-18/lib/Api/FakeApi.php
index bc23dc9481f..360a3b16f4c 100644
--- a/samples/client/petstore/php/psr-18/lib/Api/FakeApi.php
+++ b/samples/client/petstore/php/psr-18/lib/Api/FakeApi.php
@@ -4002,39 +4002,39 @@ class FakeApi
 
         // form params
         if ($integer !== null) {
-            $formParams['integer'] = ObjectSerializer::toFormValue($integer);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('integer', $integer));
         }
         // form params
         if ($int32 !== null) {
-            $formParams['int32'] = ObjectSerializer::toFormValue($int32);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('int32', $int32));
         }
         // form params
         if ($int64 !== null) {
-            $formParams['int64'] = ObjectSerializer::toFormValue($int64);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('int64', $int64));
         }
         // form params
         if ($number !== null) {
-            $formParams['number'] = ObjectSerializer::toFormValue($number);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('number', $number));
         }
         // form params
         if ($float !== null) {
-            $formParams['float'] = ObjectSerializer::toFormValue($float);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('float', $float));
         }
         // form params
         if ($double !== null) {
-            $formParams['double'] = ObjectSerializer::toFormValue($double);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('double', $double));
         }
         // form params
         if ($string !== null) {
-            $formParams['string'] = ObjectSerializer::toFormValue($string);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('string', $string));
         }
         // form params
         if ($pattern_without_delimiter !== null) {
-            $formParams['pattern_without_delimiter'] = ObjectSerializer::toFormValue($pattern_without_delimiter);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('pattern_without_delimiter', $pattern_without_delimiter));
         }
         // form params
         if ($byte !== null) {
-            $formParams['byte'] = ObjectSerializer::toFormValue($byte);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('byte', $byte));
         }
         // form params
         if ($binary !== null) {
@@ -4043,26 +4043,26 @@ class FakeApi
             $paramFiles = is_array($binary) ? $binary : [$binary];
             foreach ($paramFiles as $paramFile) {
                 $formParams['binary'][] = \GuzzleHttp\Psr7\try_fopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('binary', $paramFile)['binary'],
                     'rb'
                 );
             }
         }
         // form params
         if ($date !== null) {
-            $formParams['date'] = ObjectSerializer::toFormValue($date);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('date', $date));
         }
         // form params
         if ($date_time !== null) {
-            $formParams['dateTime'] = ObjectSerializer::toFormValue($date_time);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('dateTime', $date_time));
         }
         // form params
         if ($password !== null) {
-            $formParams['password'] = ObjectSerializer::toFormValue($password);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('password', $password));
         }
         // form params
         if ($callback !== null) {
-            $formParams['callback'] = ObjectSerializer::toFormValue($callback);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('callback', $callback));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -4371,11 +4371,11 @@ class FakeApi
 
         // form params
         if ($enum_form_string_array !== null) {
-            $formParams['enum_form_string_array'] = ObjectSerializer::toFormValue($enum_form_string_array);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('enum_form_string_array', $enum_form_string_array));
         }
         // form params
         if ($enum_form_string !== null) {
-            $formParams['enum_form_string'] = ObjectSerializer::toFormValue($enum_form_string);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('enum_form_string', $enum_form_string));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -5317,11 +5317,11 @@ class FakeApi
 
         // form params
         if ($param !== null) {
-            $formParams['param'] = ObjectSerializer::toFormValue($param);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('param', $param));
         }
         // form params
         if ($param2 !== null) {
-            $formParams['param2'] = ObjectSerializer::toFormValue($param2);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('param2', $param2));
         }
 
         $headers = $this->headerSelector->selectHeaders(
diff --git a/samples/client/petstore/php/psr-18/lib/Api/PetApi.php b/samples/client/petstore/php/psr-18/lib/Api/PetApi.php
index 09faf067888..29b35822dba 100644
--- a/samples/client/petstore/php/psr-18/lib/Api/PetApi.php
+++ b/samples/client/petstore/php/psr-18/lib/Api/PetApi.php
@@ -1818,11 +1818,11 @@ class PetApi
 
         // form params
         if ($name !== null) {
-            $formParams['name'] = ObjectSerializer::toFormValue($name);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('name', $name));
         }
         // form params
         if ($status !== null) {
-            $formParams['status'] = ObjectSerializer::toFormValue($status);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('status', $status));
         }
 
         $headers = $this->headerSelector->selectHeaders(
@@ -2094,7 +2094,7 @@ class PetApi
 
         // form params
         if ($additional_metadata !== null) {
-            $formParams['additionalMetadata'] = ObjectSerializer::toFormValue($additional_metadata);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('additionalMetadata', $additional_metadata));
         }
         // form params
         if ($file !== null) {
@@ -2103,7 +2103,7 @@ class PetApi
             $paramFiles = is_array($file) ? $file : [$file];
             foreach ($paramFiles as $paramFile) {
                 $formParams['file'][] = \GuzzleHttp\Psr7\try_fopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('file', $paramFile)['file'],
                     'rb'
                 );
             }
@@ -2384,7 +2384,7 @@ class PetApi
 
         // form params
         if ($additional_metadata !== null) {
-            $formParams['additionalMetadata'] = ObjectSerializer::toFormValue($additional_metadata);
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('additionalMetadata', $additional_metadata));
         }
         // form params
         if ($required_file !== null) {
@@ -2393,7 +2393,366 @@ class PetApi
             $paramFiles = is_array($required_file) ? $required_file : [$required_file];
             foreach ($paramFiles as $paramFile) {
                 $formParams['requiredFile'][] = \GuzzleHttp\Psr7\try_fopen(
-                    ObjectSerializer::toFormValue($paramFile),
+                    ObjectSerializer::toFormValue('requiredFile', $paramFile)['requiredFile'],
+                    'rb'
+                );
+            }
+        }
+
+        $headers = $this->headerSelector->selectHeaders(
+            ['application/json'],
+            'multipart/form-data',
+            $multipart
+        );
+
+        // for model (json/xml)
+        if (count($formParams) > 0) {
+            if ($multipart) {
+                $multipartContents = [];
+                foreach ($formParams as $formParamName => $formParamValue) {
+                    $formParamValueItems = is_array($formParamValue) ? $formParamValue : [$formParamValue];
+                    foreach ($formParamValueItems as $formParamValueItem) {
+                        $multipartContents[] = [
+                            'name' => $formParamName,
+                            'contents' => $formParamValueItem
+                        ];
+                    }
+                }
+                // for HTTP post (form)
+                $httpBody = new MultipartStream($multipartContents);
+
+            } elseif ($this->headerSelector->isJsonMime($headers['Content-Type'])) {
+                $httpBody = json_encode($formParams);
+
+            } else {
+                // for HTTP post (form)
+                $httpBody = ObjectSerializer::buildQuery($formParams);
+            }
+        }
+
+        // this endpoint requires OAuth (access token)
+        if ($this->config->getAccessToken() !== null) {
+            $headers['Authorization'] = 'Bearer ' . $this->config->getAccessToken();
+        }
+
+        $defaultHeaders = [];
+        if ($this->config->getUserAgent()) {
+            $defaultHeaders['User-Agent'] = $this->config->getUserAgent();
+        }
+
+        $headers = array_merge(
+            $defaultHeaders,
+            $headerParams,
+            $headers
+        );
+
+        $operationHost = $this->config->getHost();
+
+        $uri = $this->createUri($operationHost, $resourcePath, $queryParams);
+
+        return $this->createRequest('POST', $uri, $headers, $httpBody);
+    }
+
+    /**
+     * Operation uploadImageFullFormData
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name name (required)
+     * @param  string[] $photo_urls photo_urls (required)
+     * @param  int $id id (optional)
+     * @param  \OpenAPI\Client\Model\Category $category category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[] $tags tags (optional)
+     * @param  string $status pet status in the store (optional)
+     * @param  \SplFileObject $file file to upload (optional)
+     * @param  \SplFileObject[] $multiple_files multiple_files (optional)
+     *
+     * @throws \OpenAPI\Client\ApiException on non-2xx response
+     * @throws \InvalidArgumentException
+     * @return \OpenAPI\Client\Model\ApiResponse
+     */
+    public function uploadImageFullFormData($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null)
+    {
+        list($response) = $this->uploadImageFullFormDataWithHttpInfo($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files);
+        return $response;
+    }
+
+    /**
+     * Operation uploadImageFullFormDataWithHttpInfo
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int $id (optional)
+     * @param  \OpenAPI\Client\Model\Category $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[] $tags (optional)
+     * @param  string $status pet status in the store (optional)
+     * @param  \SplFileObject $file file to upload (optional)
+     * @param  \SplFileObject[] $multiple_files (optional)
+     *
+     * @throws \OpenAPI\Client\ApiException on non-2xx response
+     * @throws \InvalidArgumentException
+     * @return array of \OpenAPI\Client\Model\ApiResponse, HTTP status code, HTTP response headers (array of strings)
+     */
+    public function uploadImageFullFormDataWithHttpInfo($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null)
+    {
+        $request = $this->uploadImageFullFormDataRequest($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files);
+
+        try {
+            try {
+                $response = $this->httpClient->sendRequest($request);
+            } catch (HttpException $e) {
+                $response = $e->getResponse();
+                throw new ApiException(
+                    sprintf(
+                        '[%d] Error connecting to the API (%s)',
+                        $response->getStatusCode(),
+                        (string) $request->getUri()
+                    ),
+                    $request,
+                    $response,
+                    $e
+                );
+            } catch (ClientExceptionInterface $e) {
+                throw new ApiException(
+                    "[{$e->getCode()}] {$e->getMessage()}",
+                    $request,
+                    null,
+                    $e
+                );
+            }
+
+            $statusCode = $response->getStatusCode();
+
+            switch($statusCode) {
+                case 200:
+                    if ('\OpenAPI\Client\Model\ApiResponse' === '\SplFileObject') {
+                        $content = $response->getBody(); //stream goes to serializer
+                    } else {
+                        $content = (string) $response->getBody();
+                    }
+
+                    return [
+                        ObjectSerializer::deserialize($content, '\OpenAPI\Client\Model\ApiResponse', []),
+                        $response->getStatusCode(),
+                        $response->getHeaders()
+                    ];
+            }
+
+            $returnType = '\OpenAPI\Client\Model\ApiResponse';
+            if ($returnType === '\SplFileObject') {
+                $content = $response->getBody(); //stream goes to serializer
+            } else {
+                $content = (string) $response->getBody();
+            }
+
+            return [
+                ObjectSerializer::deserialize($content, $returnType, []),
+                $response->getStatusCode(),
+                $response->getHeaders()
+            ];
+
+        } catch (ApiException $e) {
+            switch ($e->getCode()) {
+                case 200:
+                    $data = ObjectSerializer::deserialize(
+                        $e->getResponseBody(),
+                        '\OpenAPI\Client\Model\ApiResponse',
+                        $e->getResponseHeaders()
+                    );
+                    $e->setResponseObject($data);
+                    break;
+            }
+            throw $e;
+        }
+    }
+
+    /**
+     * Operation uploadImageFullFormDataAsync
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int $id (optional)
+     * @param  \OpenAPI\Client\Model\Category $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[] $tags (optional)
+     * @param  string $status pet status in the store (optional)
+     * @param  \SplFileObject $file file to upload (optional)
+     * @param  \SplFileObject[] $multiple_files (optional)
+     *
+     * @throws \InvalidArgumentException
+     * @return Promise
+     */
+    public function uploadImageFullFormDataAsync($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null)
+    {
+        return $this->uploadImageFullFormDataAsyncWithHttpInfo($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files)
+            ->then(
+                function ($response) {
+                    return $response[0];
+                }
+            );
+    }
+
+    /**
+     * Operation uploadImageFullFormDataAsyncWithHttpInfo
+     *
+     * uploads an image attached to a Pet object as formdata
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int $id (optional)
+     * @param  \OpenAPI\Client\Model\Category $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[] $tags (optional)
+     * @param  string $status pet status in the store (optional)
+     * @param  \SplFileObject $file file to upload (optional)
+     * @param  \SplFileObject[] $multiple_files (optional)
+     *
+     * @throws \InvalidArgumentException
+     * @return Promise
+     */
+    public function uploadImageFullFormDataAsyncWithHttpInfo($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null)
+    {
+        $returnType = '\OpenAPI\Client\Model\ApiResponse';
+        $request = $this->uploadImageFullFormDataRequest($pet_id, $name, $photo_urls, $id, $category, $tags, $status, $file, $multiple_files);
+
+        return $this->httpAsyncClient->sendAsyncRequest($request)
+            ->then(
+                function ($response) use ($returnType) {
+                    if ($returnType === '\SplFileObject') {
+                        $content = $response->getBody(); //stream goes to serializer
+                    } else {
+                        $content = (string) $response->getBody();
+                    }
+
+                    return [
+                        ObjectSerializer::deserialize($content, $returnType, []),
+                        $response->getStatusCode(),
+                        $response->getHeaders()
+                    ];
+                },
+                function (HttpException $exception) {
+                    $response = $exception->getResponse();
+                    $statusCode = $response->getStatusCode();
+                    throw new ApiException(
+                        sprintf(
+                            '[%d] Error connecting to the API (%s)',
+                            $statusCode,
+                            $exception->getRequest()->getUri()
+                        ),
+                        $exception->getRequest(),
+                        $exception->getResponse(),
+                        $exception
+                    );
+                }
+            );
+    }
+
+    /**
+     * Create request for operation 'uploadImageFullFormData'
+     *
+     * @param  int $pet_id ID of pet to update (required)
+     * @param  string $name (required)
+     * @param  string[] $photo_urls (required)
+     * @param  int $id (optional)
+     * @param  \OpenAPI\Client\Model\Category $category (optional)
+     * @param  \OpenAPI\Client\Model\Tag[] $tags (optional)
+     * @param  string $status pet status in the store (optional)
+     * @param  \SplFileObject $file file to upload (optional)
+     * @param  \SplFileObject[] $multiple_files (optional)
+     *
+     * @throws \InvalidArgumentException
+     * @return RequestInterface
+     */
+    public function uploadImageFullFormDataRequest($pet_id, $name, $photo_urls, $id = null, $category = null, $tags = null, $status = null, $file = null, $multiple_files = null)
+    {
+        // verify the required parameter 'pet_id' is set
+        if ($pet_id === null || (is_array($pet_id) && count($pet_id) === 0)) {
+            throw new \InvalidArgumentException(
+                'Missing the required parameter $pet_id when calling uploadImageFullFormData'
+            );
+        }
+        // verify the required parameter 'name' is set
+        if ($name === null || (is_array($name) && count($name) === 0)) {
+            throw new \InvalidArgumentException(
+                'Missing the required parameter $name when calling uploadImageFullFormData'
+            );
+        }
+        // verify the required parameter 'photo_urls' is set
+        if ($photo_urls === null || (is_array($photo_urls) && count($photo_urls) === 0)) {
+            throw new \InvalidArgumentException(
+                'Missing the required parameter $photo_urls when calling uploadImageFullFormData'
+            );
+        }
+
+
+        $resourcePath = '/pet/{petId}/uploadImageFullFormData';
+        $formParams = [];
+        $queryParams = [];
+        $headerParams = [];
+        $httpBody = null;
+        $multipart = false;
+
+
+
+        // path params
+        if ($pet_id !== null) {
+            $resourcePath = str_replace(
+                '{' . 'petId' . '}',
+                ObjectSerializer::toPathValue($pet_id),
+                $resourcePath
+            );
+        }
+
+        // form params
+        if ($id !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('id', $id));
+        }
+        // form params
+        if ($category !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('category', $category));
+        }
+        // form params
+        if ($name !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('name', $name));
+        }
+        // form params
+        if ($photo_urls !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('photoUrls', $photo_urls));
+        }
+        // form params
+        if ($tags !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('tags', $tags));
+        }
+        // form params
+        if ($status !== null) {
+            $formParams = array_merge($formParams, ObjectSerializer::toFormValue('status', $status));
+        }
+        // form params
+        if ($file !== null) {
+            $multipart = true;
+            $formParams['file'] = [];
+            $paramFiles = is_array($file) ? $file : [$file];
+            foreach ($paramFiles as $paramFile) {
+                $formParams['file'][] = \GuzzleHttp\Psr7\try_fopen(
+                    ObjectSerializer::toFormValue('file', $paramFile)['file'],
+                    'rb'
+                );
+            }
+        }
+        // form params
+        if ($multiple_files !== null) {
+            $multipart = true;
+            $formParams['multiple_files'] = [];
+            $paramFiles = is_array($multiple_files) ? $multiple_files : [$multiple_files];
+            foreach ($paramFiles as $paramFile) {
+                $formParams['multiple_files'][] = \GuzzleHttp\Psr7\try_fopen(
+                    ObjectSerializer::toFormValue('multiple_files', $paramFile)['multiple_files'],
                     'rb'
                 );
             }
diff --git a/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php b/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php
index 440a243a41a..3e1cbe8778b 100644
--- a/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php
+++ b/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php
@@ -28,6 +28,7 @@
 
 namespace OpenAPI\Client;
 
+use ArrayAccess;
 use GuzzleHttp\Psr7\Utils;
 use OpenAPI\Client\Model\ModelInterface;
 
@@ -324,20 +325,31 @@ class ObjectSerializer
     }
 
     /**
-     * Take value and turn it into a string suitable for inclusion in
+     * Take value and turn it into an array suitable for inclusion in
      * the http body (form parameter). If it's a string, pass through unchanged
      * If it's a datetime object, format it in ISO8601
      *
-     * @param string|\SplFileObject $value the value of the form parameter
+     * @param string|bool|array|DateTime|ArrayAccess|\SplFileObject $value the value of the form parameter
      *
-     * @return string the form string
+     * @return array [key => value] of formdata
      */
-    public static function toFormValue($value)
+    public static function toFormValue(string $key, mixed $value)
     {
         if ($value instanceof \SplFileObject) {
-            return $value->getRealPath();
+            return [$key => $value->getRealPath()];
+        } elseif (is_array($value) || $value instanceof ArrayAccess) {
+            $flattened = [];
+            $result = [];
+
+            self::flattenArray(json_decode(json_encode($value), true), $flattened);
+
+            foreach ($flattened as $k => $v) {
+                $result["{$key}{$k}"] = self::toString($v);
+            }
+
+            return $result;
         } else {
-            return self::toString($value);
+            return [$key => self::toString($value)];
         }
     }
 
@@ -614,4 +626,81 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * @param \ArrayAccess|array $source
+     *
+     * credit: https://github.com/FranBar1966/FlatPHP
+     */
+    private static function flattenArray(
+        mixed $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        /**
+         * array_is_list only in PHP >= 8.1
+         *
+         * credit: https://www.php.net/manual/en/function.array-is-list.php#127044
+         */
+        if (!function_exists('array_is_list')) {
+            function array_is_list(array $array)
+            {
+                $i = -1;
+
+                foreach ($array as $k => $v) {
+                    ++$i;
+                    if ($k !== $i) {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flattenArray($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }