How can I set the PHP version in PHPStorm?

So, you mean to highlight all pieces of code that will not work with the PHP version you are writing it in, right? That can be done here:

Preferences -> Languages & Frameworks > PHP

and the same on Mac:

PhpStorm -> Preferences (⌘,) -> Languages & Frameworks > PHP

Or with newer versions of PhpStorm in Windows:

File -> Settings -> Languages & Frameworks > PHP

then select your PHP version, for example, 7.0

This is very useful when your local system runs PHP 7.0, for example, but production is running PHP 5.5.

That way PhpStorm will warn you which parts will not work in production, show proper hints during writing code according to the selected version, etc.