[php] Set $multipart to true if content-type is multipart/form-data (OpenAPITools#5536) (OpenAPITools#18991)

* [php] Set $multipart to true if content-type is multipart/form-data (OpenAPITools#5536)

Related: OpenAPITools#5536

* [php] Use isMultipart variable

---------

Co-authored-by: Simon Hammes <[email protected]>

Author: simonhammes

modules/openapi-generator/src/main/resources/php/api.mustache

@@ -688,6 +688,9 @@ use {{invokerPackage}}\ObjectSerializer;
         }
         {{/formParams}}
 
+        {{#isMultipart}}
+        $multipart = true;
+        {{/isMultipart}}
         $headers = $this->headerSelector->selectHeaders(
             [{{#produces}}'{{{mediaType}}}', {{/produces}}],
             $contentType,

samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php

@@ -2571,6 +2571,7 @@ public function uploadFileRequest($pet_id, $additional_metadata = null, $file =
             }
         }
 
+        $multipart = true;
         $headers = $this->headerSelector->selectHeaders(
             ['application/json', ],
             $contentType,
@@ -2917,6 +2918,7 @@ public function uploadFileWithRequiredFileRequest($pet_id, $required_file, $addi
             }
         }
 
+        $multipart = true;
         $headers = $this->headerSelector->selectHeaders(
             ['application/json', ],
             $contentType,