The first worldwide PHP TestFest will be held in May 2008. The idea of the TestFest is simply to write more tests for the PHP language - this helps the PHP maintainers to test fixes and implementation of new features. So it's a good thing.
One of the little known facts about writing tests for PHP is that if you can write PHP you can write test cases. You don't need to be able to write C (although being able to read it does help). For PHP developers this is a great way to get involved in one of the most the successful open source projects of all time!
Many local PHP user groups are taking part in the TestFest, check the PHP Wiki for details and subscribe to the PHP QA list to hear about plans for other events. In most cases the user groups are running events aimed at helping people to learn how to write tests.
When new tests are written as part of the TestFest they will be reviewed by PHP test mentors. If the tests are good they'll become part of the PHP code base.
I did hear a rumour that there might be a limited edition T-shirt going to people who have five new tests accepted....there was also a whisper about ElePHPants....