The PHPNW user group (based out of Manchester, United Kingdom) will be taking part in this year's PHP TestFest, an event to encourage more complete code coverage of the PHP language through user-contributed phpt tests. Lorna Mitchell has more about their involvement in this recent post to her blog.
Our event will be on 9th May - you can sign up to attend (limited places so do let us know) on the upcoming page for this event: http://upcoming.yahoo.com/event/2299548/. The idea behind Test Fest is twofold: firstly, to give an approachable way for PHP developers to contribute back to the core - we'll learn how to write tests on the day and can continue to do so after the event of course! Secondly, PHP itself gets better tests, and the language itself gets better. All in all, I can't see a reason not to do it!
If you're not in the area of Manchester (or even in the same country) you still might have a group near you participating - check out this page on the PHP.net wiki to find one close (ten countries are represented so far).