Update implementation

Author: kocsismate

Zend/tests/enum/json_encode.phpt

@@ -18,7 +18,7 @@ enum StringFoo: string {
 enum CustomFoo implements JsonSerializable {
     case Bar;
 
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return 'Custom ' . $this->name;
     }
 }

Zend/tests/iterator_key_by_ref.phpt

@@ -3,7 +3,7 @@ Iterator::key() with by-ref return
 --FILE--
 <?php
 class Test extends ArrayIterator {
-    function &key() {
+    function &key(): mixed {
         return $foo;
     }
 }