diff --git a/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache b/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
index c650ac86df5..b076414630c 100644
--- a/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
+++ b/modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache
@@ -324,7 +324,7 @@ class ObjectSerializer
      *
      * @return array [key => value] of formdata
      */
-    public static function toFormValue(string $key, mixed $value)
+    public static function toFormValue(string $key, $value)
     {
         if ($value instanceof \SplFileObject) {
             return [$key => $value->getRealPath()];
@@ -628,9 +628,9 @@ class ObjectSerializer
      * credit: https://github.com/FranBar1966/FlatPHP
      */
     private static function flattenArray(
-        mixed $source,
+        $source,
         array &$destination,
-        string $start = '',
+        string $start = ''
     ) {
         $opt = [
             'prefix'          => '[',
diff --git a/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php b/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
index d6d1fa941d6..a144eb7641b 100644
--- a/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
+++ b/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php
@@ -333,7 +333,7 @@ class ObjectSerializer
      *
      * @return array [key => value] of formdata
      */
-    public static function toFormValue(string $key, mixed $value)
+    public static function toFormValue(string $key, $value)
     {
         if ($value instanceof \SplFileObject) {
             return [$key => $value->getRealPath()];
@@ -637,9 +637,9 @@ class ObjectSerializer
      * credit: https://github.com/FranBar1966/FlatPHP
      */
     private static function flattenArray(
-        mixed $source,
+        $source,
         array &$destination,
-        string $start = '',
+        string $start = ''
     ) {
         $opt = [
             'prefix'          => '[',
diff --git a/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php b/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php
index d6d1fa941d6..a144eb7641b 100644
--- a/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php
+++ b/samples/client/petstore/php/psr-18/lib/ObjectSerializer.php
@@ -333,7 +333,7 @@ class ObjectSerializer
      *
      * @return array [key => value] of formdata
      */
-    public static function toFormValue(string $key, mixed $value)
+    public static function toFormValue(string $key, $value)
     {
         if ($value instanceof \SplFileObject) {
             return [$key => $value->getRealPath()];
@@ -637,9 +637,9 @@ class ObjectSerializer
      * credit: https://github.com/FranBar1966/FlatPHP
      */
     private static function flattenArray(
-        mixed $source,
+        $source,
         array &$destination,
-        string $start = '',
+        string $start = ''
     ) {
         $opt = [
             'prefix'          => '[',