On the php[architect] site they've posted an article from the September 2017 issue - the "Community Corner" from James Titcumb all about the return of PHP TestFest.

There is a new movement in the PHP community! Well, it isn’t exactly new, but an event called PHP TestFest is back. PHP TestFest is a global event organized by the PHP community at large, where an effort is made each time to increase the test coverage of the PHP engine itself. Around eight or nine years ago, user groups around the world coordinated to organize events spanning a few months each year, with some groups continuing events annually for several years after.

The article goes on to talk about some of the reasoning behind the TestFest and how you can help to set up an event for a local group to contribute. It then goes through the format of tests, creating new ones and how to execute them to test the result. You can find out more about PHP TestFest and how you can get involved on the main PHP TestFest site.