public static function isKeyword($str, $isReserved = false)
    {
        $str = strtoupper($str);

        if (isset(static::$KEYWORDS[$str])) {
            if ($isReserved && ! (static::$KEYWORDS[$str] & Token::FLAG_KEYWORD_RESERVED)) {
                return null;
            }

            return static::$KEYWORDS[$str];
        }

        return null;
    }