diff --git a/jwt.php b/jwt.php
index 3c4a429..4445f2c 100644
--- a/jwt.php
+++ b/jwt.php
@@ -159,6 +159,8 @@ class jwt
   // }}}
 
   /** Return the provided string in base64 without equal at the end
+    * To be URL compliant, the slash and plus are converted to underscore and
+    * dash
     * @param string $str The string the convert in base64
     * @return string The string converted in base64
     */