public function setDebugMessage($message, $details = "")
    {
        if (empty($message)) {
            return;
        }
        
        // we keep messages in the session
        // until we are sure they are shown
        // otherwise some messages may be lost
        // because of redirection
        self::$session_vars["debug"][$message] = ["message" => $message, "details" => $details];
    }