The Yahoo-acquired site Upcoming.org has released a new (PHP 5 only) wrapper class for interfacing with their backend API. The Zend Developer Zone has the full story.
On Sept 28, 2006, Upcoming.org released it's new PHP 5 (only) wrapper class. Details can be found here and the wiki page can be found here.
The wiki page lists the prerequsites you'll need to use the wrapper (including SimpleMXL, PEAR's Cache::Lite, and the API documentation) as well as a simple "getting started" guide to making requests. There are a few issues they mention happening with the package, so keep an eye out for future updates.