update php samples

Author: wing328

samples/client/petstore/php/psr-18/lib/Api/FakeApi.php

@@ -672,7 +672,7 @@ public function fakeEnumEndpointRequest($enum_class, $enum_class_array, $enum_cl
                 // for HTTP post (form)
                 $httpBody = new MultipartStream($multipartContents);
 
-            } elseif ($headers['Content-Type'] === 'application/json') {
+            } elseif ($this->headerSelector->isJsonMime($headers['Content-Type'])) {
                 $httpBody = json_encode($formParams);
 
             } else {