Via PHP.net:
The PHP development community is proud to announce the release of PHP 5 Beta 1. Both source packages, and a Windows build are available in the Downloads Section. A list of changes can be found in the ChangeLog file.
Notable features include a new version of the Zend engine, a new XML library, and other fun stuff.
It’s a beta. Put it on a test box and play with it. Don’t put it on a production server. Don’t do it, man.