public static function isString($str)
    {
        if (strlen($str) === 0) {
            return null;
        }
        if ($str[0] === '\'') {
            return Token::FLAG_STRING_SINGLE_QUOTES;
        } elseif ($str[0] === '"') {
            return Token::FLAG_STRING_DOUBLE_QUOTES;
        }

        return null;
    }