public static function fetch_discussion_subscription($forumid, $userid = null) {
        self::fill_discussion_subscription_cache($forumid, $userid);

        if (!isset(self::$forumdiscussioncache[$userid]) || !isset(self::$forumdiscussioncache[$userid][$forumid])) {
            return array();
        }

        return self::$forumdiscussioncache[$userid][$forumid];
    }