public function saveHistory($extra = null)
    {
        if (!is_null($extra)) {
            self::$queries[] = $extra;
            self::$params[] = null;
        }

        self::$queries[] = 'Saved query-history to session.';
        self::$params[] = null;

        $key = $this->options['session_key'];
        $_SESSION[$key]['numQueries'] = self::$numQueries;
        $_SESSION[$key]['queries']    = self::$queries;
        $_SESSION[$key]['params']     = self::$params;
    }