Add PHP coding style rules and check PHP coding style

[mlocati]

No description provided.

[pierrejoye]

It seems 7.2 is failing here too

[mlocati]

It seems 7.2 is failing here too

Tests have been fixed by #213, so rebasing this PR should fix that failure. But of course since this PR doesn't include the fixes of the coding style rules, the "Check coding style" fails.
Before applying the rules, we'd need to decide if the rules I propose here are ok for you.

For example, I have a question: having a very long license related comment at the beginning of every PHP file makes the code a bit harder to be read (example).
So, what about replacing it with something like this?

/*
 * This file is part of the Pickle project.
 * @license https://github.com/FriendsOfPHP/pickle/blob/master/LICENSE
 */

[mlocati]

PS: you can see the changes that would fix the proposed rules by seeing the output of the "Check coding style" step (this one at the moment).