diff --git a/php/src/Utils.php b/php/src/Utils.php
index 13ca84f..654bfc4 100644
--- a/php/src/Utils.php
+++ b/php/src/Utils.php
@@ -267,7 +267,7 @@ public static function equalNumber($left, $right)
      *
      * @return bool if unset, return true
      */
-    public static function isUnset(&$value = null)
+    public static function isUnset($value = null)
     {
         return !isset($value) || null === $value;
     }