private static function make_checksum() {
        $ret = "";
        $_SESSION[__SESSION_time_var] = \AsyncWeb\Date\Time::get();
        if (isset($_SESSION) && is_array($_SESSION)) foreach ($_SESSION as $s => $v) {
            if ($s != __SESSION_checksum_var) {
                $ret = md5($ret . $s);
            }
        }
        Session::$checksum = $ret;
        return $ret;
    }