This function matches strings with underscores, because the \w escape sequence allows underscores:

A "word" character is any letter or digit or the underscore character, that is, any character which can be part of a Perl "word".

which are against this condition in the kata's description:

• No whitespaces/underscore

For example, this regex expression matches the string "string_with_underscores".

Edit: \d is unneeded in the PCRE expression. The \w escape character already covers digits.

Literally forgot I could have used a ternery operator :facepalm:

In addition to what @Dec0y-jb said, It's also takes advantage of what PHP considers "truthy" and "falsey". To learn more about what PHP considers "true", see the truth table in the documentation.
http://php.net/manual/en/types.comparisons.php

Here, the code array_sum($a) % 2 returns an integer number. If it returns any number other than 0, the ternary operator sees it as a "true" condition, thereby returning the first value. However, if it returns exactly 0, the ternary operator sees it as a "false" condition, returning the second value.

I've never understood these ternery operators :/

This does not seem to account for the case in which an empty string is passed in.