diff --git a/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache b/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
index b9993c0410c..3ab026909ec 100644
--- a/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
+++ b/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
@@ -19,6 +19,8 @@
 
 namespace {{invokerPackage}};
 
+use {{modelPackage}}\ModelInterface;
+
 /**
  * ObjectSerializer Class Doc Comment
  *
@@ -51,19 +53,25 @@ class ObjectSerializer
             return $data;
         } elseif (is_object($data)) {
             $values = [];
-            $formats = $data::openAPIFormats();
-            foreach ($data::openAPITypes() as $property => $openAPIType) {
-                $getter = $data::getters()[$property];
-                $value = $data->$getter();
-                if ($value !== null
-                    && !in_array($openAPIType, [{{&primitives}}], true)
-                    && method_exists($openAPIType, 'getAllowableEnumValues')
-                    && !in_array($value, $openAPIType::getAllowableEnumValues(), true)) {
-                    $imploded = implode("', '", $openAPIType::getAllowableEnumValues());
-                    throw new \InvalidArgumentException("Invalid value for enum '$openAPIType', must be one of: '$imploded'");
+            if ($data instanceof ModelInterface) {
+                $formats = $data::openAPIFormats();
+                foreach ($data::openAPITypes() as $property => $openAPIType) {
+                    $getter = $data::getters()[$property];
+                    $value = $data->$getter();
+                    if ($value !== null
+                        && !in_array($openAPIType, [{{&primitives}}], true)
+                        && method_exists($openAPIType, 'getAllowableEnumValues')
+                        && !in_array($value, $openAPIType::getAllowableEnumValues(), true)) {
+                        $imploded = implode("', '", $openAPIType::getAllowableEnumValues());
+                        throw new \InvalidArgumentException("Invalid value for enum '$openAPIType', must be one of: '$imploded'");
+                    }
+                    if ($value !== null) {
+                        $values[$data::attributeMap()[$property]] = self::sanitizeForSerialization($value, $openAPIType, $formats[$property]);
+                    }
                 }
-                if ($value !== null) {
-                    $values[$data::attributeMap()[$property]] = self::sanitizeForSerialization($value, $openAPIType, $formats[$property]);
+            } else {
+                foreach($data as $property => $value) {
+                    $values[$property] = self::sanitizeForSerialization($value);
                 }
             }
             return (object)$values;
diff --git a/modules/openapi-generator/src/main/resources/php/api.mustache b/modules/openapi-generator/src/main/resources/php/api.mustache
index 7ecf4fc9286..4f1fc87045e 100644
--- a/modules/openapi-generator/src/main/resources/php/api.mustache
+++ b/modules/openapi-generator/src/main/resources/php/api.mustache
@@ -444,10 +444,10 @@ use {{invokerPackage}}\ObjectSerializer;
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
index 644aaca9230..07d87634b52 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
@@ -307,10 +307,10 @@ class AnotherFakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
index c8691f7e932..6ee66f33c0f 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
@@ -297,10 +297,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -554,10 +554,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -811,10 +811,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1068,10 +1068,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1283,10 +1283,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1513,10 +1513,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1780,10 +1780,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2183,10 +2183,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2470,10 +2470,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2689,10 +2689,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2924,10 +2924,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
index 2870a4c9050..0b2de02bc7c 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
@@ -307,10 +307,10 @@ class FakeClassnameTags123Api
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
index 40d4cb8e1b7..ff7d6f689ec 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
@@ -259,10 +259,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -496,10 +496,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -771,10 +771,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1046,10 +1046,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1322,10 +1322,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1546,10 +1546,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1792,10 +1792,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2087,10 +2087,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2388,10 +2388,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
index f12f7270d96..7a4dc235a37 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
@@ -264,10 +264,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -517,10 +517,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -801,10 +801,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1068,10 +1068,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
index 492c68ff573..b3fecdc2b33 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
@@ -259,10 +259,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -478,10 +478,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -697,10 +697,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -921,10 +921,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1193,10 +1193,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1476,10 +1476,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1681,10 +1681,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1919,10 +1919,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php b/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
index 3592527a968..d7b254c954e 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
@@ -29,6 +29,8 @@
 
 namespace OpenAPI\Client;
 
+use OpenAPI\Client\Model\ModelInterface;
+
 /**
  * ObjectSerializer Class Doc Comment
  *
@@ -61,19 +63,25 @@ class ObjectSerializer
             return $data;
         } elseif (is_object($data)) {
             $values = [];
-            $formats = $data::openAPIFormats();
-            foreach ($data::openAPITypes() as $property => $openAPIType) {
-                $getter = $data::getters()[$property];
-                $value = $data->$getter();
-                if ($value !== null
-                    && !in_array($openAPIType, ['DateTime', 'bool', 'boolean', 'byte', 'double', 'float', 'int', 'integer', 'mixed', 'number', 'object', 'string', 'void'], true)
-                    && method_exists($openAPIType, 'getAllowableEnumValues')
-                    && !in_array($value, $openAPIType::getAllowableEnumValues(), true)) {
-                    $imploded = implode("', '", $openAPIType::getAllowableEnumValues());
-                    throw new \InvalidArgumentException("Invalid value for enum '$openAPIType', must be one of: '$imploded'");
+            if ($data instanceof ModelInterface) {
+                $formats = $data::openAPIFormats();
+                foreach ($data::openAPITypes() as $property => $openAPIType) {
+                    $getter = $data::getters()[$property];
+                    $value = $data->$getter();
+                    if ($value !== null
+                        && !in_array($openAPIType, ['DateTime', 'bool', 'boolean', 'byte', 'double', 'float', 'int', 'integer', 'mixed', 'number', 'object', 'string', 'void'], true)
+                        && method_exists($openAPIType, 'getAllowableEnumValues')
+                        && !in_array($value, $openAPIType::getAllowableEnumValues(), true)) {
+                        $imploded = implode("', '", $openAPIType::getAllowableEnumValues());
+                        throw new \InvalidArgumentException("Invalid value for enum '$openAPIType', must be one of: '$imploded'");
+                    }
+                    if ($value !== null) {
+                        $values[$data::attributeMap()[$property]] = self::sanitizeForSerialization($value, $openAPIType, $formats[$property]);
+                    }
                 }
-                if ($value !== null) {
-                    $values[$data::attributeMap()[$property]] = self::sanitizeForSerialization($value, $openAPIType, $formats[$property]);
+            } else {
+                foreach($data as $property => $value) {
+                    $values[$property] = self::sanitizeForSerialization($value);
                 }
             }
             return (object)$values;
diff --git a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
index 644aaca9230..07d87634b52 100644
--- a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
+++ b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
@@ -307,10 +307,10 @@ class AnotherFakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
index 8e18423c90a..6cc26632fe8 100644
--- a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
+++ b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeApi.php
@@ -297,10 +297,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -554,10 +554,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -811,10 +811,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1068,10 +1068,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1283,10 +1283,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1513,10 +1513,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1780,10 +1780,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2183,10 +2183,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2470,10 +2470,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2689,10 +2689,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2924,10 +2924,10 @@ class FakeApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
index 2870a4c9050..0b2de02bc7c 100644
--- a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
+++ b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/FakeClassnameTags123Api.php
@@ -307,10 +307,10 @@ class FakeClassnameTags123Api
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
index 40d4cb8e1b7..ff7d6f689ec 100644
--- a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
+++ b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php
@@ -259,10 +259,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -496,10 +496,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -771,10 +771,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1046,10 +1046,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1322,10 +1322,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1546,10 +1546,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1792,10 +1792,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2087,10 +2087,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -2388,10 +2388,10 @@ class PetApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
index 44c9421b2cb..c23fc73cb4f 100644
--- a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
+++ b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/StoreApi.php
@@ -264,10 +264,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -517,10 +517,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -801,10 +801,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1068,10 +1068,10 @@ class StoreApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
index 5f3227b2878..a30fdc19bbf 100644
--- a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
+++ b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/UserApi.php
@@ -259,10 +259,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -478,10 +478,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -697,10 +697,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -921,10 +921,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1193,10 +1193,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1476,10 +1476,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1681,10 +1681,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
@@ -1919,10 +1919,10 @@ class UserApi
         // for model (json/xml)
         if (isset($_tempBody)) {
             // $_tempBody is the method argument, if present
-            $httpBody = $_tempBody;
-            // \stdClass has no __toString(), so we should encode it manually
-            if ($httpBody instanceof \stdClass && $headers['Content-Type'] === 'application/json') {
-                $httpBody = \GuzzleHttp\json_encode($httpBody);
+            if ($headers['Content-Type'] === 'application/json') {
+                $httpBody = \GuzzleHttp\json_encode(ObjectSerializer::sanitizeForSerialization($_tempBody));
+            } else {
+                $httpBody = $_tempBody;
             }
         } elseif (count($formParams) > 0) {
             if ($multipart) {
diff --git a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
index 3592527a968..d7b254c954e 100644
--- a/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
+++ b/samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
@@ -29,6 +29,8 @@
 
 namespace OpenAPI\Client;
 
+use OpenAPI\Client\Model\ModelInterface;
+
 /**
  * ObjectSerializer Class Doc Comment
  *
@@ -61,19 +63,25 @@ class ObjectSerializer
             return $data;
         } elseif (is_object($data)) {
             $values = [];
-            $formats = $data::openAPIFormats();
-            foreach ($data::openAPITypes() as $property => $openAPIType) {
-                $getter = $data::getters()[$property];
-                $value = $data->$getter();
-                if ($value !== null
-                    && !in_array($openAPIType, ['DateTime', 'bool', 'boolean', 'byte', 'double', 'float', 'int', 'integer', 'mixed', 'number', 'object', 'string', 'void'], true)
-                    && method_exists($openAPIType, 'getAllowableEnumValues')
-                    && !in_array($value, $openAPIType::getAllowableEnumValues(), true)) {
-                    $imploded = implode("', '", $openAPIType::getAllowableEnumValues());
-                    throw new \InvalidArgumentException("Invalid value for enum '$openAPIType', must be one of: '$imploded'");
+            if ($data instanceof ModelInterface) {
+                $formats = $data::openAPIFormats();
+                foreach ($data::openAPITypes() as $property => $openAPIType) {
+                    $getter = $data::getters()[$property];
+                    $value = $data->$getter();
+                    if ($value !== null
+                        && !in_array($openAPIType, ['DateTime', 'bool', 'boolean', 'byte', 'double', 'float', 'int', 'integer', 'mixed', 'number', 'object', 'string', 'void'], true)
+                        && method_exists($openAPIType, 'getAllowableEnumValues')
+                        && !in_array($value, $openAPIType::getAllowableEnumValues(), true)) {
+                        $imploded = implode("', '", $openAPIType::getAllowableEnumValues());
+                        throw new \InvalidArgumentException("Invalid value for enum '$openAPIType', must be one of: '$imploded'");
+                    }
+                    if ($value !== null) {
+                        $values[$data::attributeMap()[$property]] = self::sanitizeForSerialization($value, $openAPIType, $formats[$property]);
+                    }
                 }
-                if ($value !== null) {
-                    $values[$data::attributeMap()[$property]] = self::sanitizeForSerialization($value, $openAPIType, $formats[$property]);
+            } else {
+                foreach($data as $property => $value) {
+                    $values[$property] = self::sanitizeForSerialization($value);
                 }
             }
             return (object)$values;