diff --git a/phpBB/includes/functions.php b/phpBB/includes/functions.php
index b4709544cf..ce9e655c11 100644
--- a/phpBB/includes/functions.php
+++ b/phpBB/includes/functions.php
@@ -1460,7 +1460,7 @@ function tracking_unserialize($string, $max_depth = 3)
 * @param string $url The url the session id needs to be appended to (can have params)
 * @param mixed $params String or array of additional url parameters
 * @param bool $is_amp Is url using & (true) or & (false)
-* @param string $session_id Possibility to use a custom session id instead of the global one
+* @param string $session_id Possibility to use a custom session id instead of the global one; deprecated as of 4.0.0-a1
 * @param bool $is_route Is url generated by a route.
 *
 * @return string The corrected url.