diff --git a/include/helper.php b/include/helper.php
index 24e0173..45e31ac 100644
--- a/include/helper.php
+++ b/include/helper.php
@@ -10,7 +10,7 @@
     die();
   }
 
-  function uuid () {
+  function uuid (): string {
     return sprintf(
       '%04x%04x-%04x-%04x-%04x-%04x%04x%04x',
       mt_rand(0, 0xffff), mt_rand(0, 0xffff), // 32bitlow