diff --git a/src/includes/common.php b/src/includes/common.php
index a24f7b2..cc34e8e 100644
--- a/src/includes/common.php
+++ b/src/includes/common.php
@@ -177,9 +177,8 @@ function isValidSignature(string $signature, string $id, string $key, string $sa
 }
 
 
-function randomString(int $len = 16) : string
+function randomString(int $len = 16, string $chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ') : string
 {
-    $chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
     $a = [];
     $max = strlen($chars) - 1;
     for ($i = 0; $i < $len; $i++) {