On PHP.net today they've announced the tagging and release of the latest alpha for the PHP 5.5.0 series - PHP 5.5.0 alpha4:
The PHP development team announces the release of PHP 5.5.0alpha4. This release fixe some bugs from alpha3 and add some new features. All users of PHP are encouraged to test this version carefully, and report any bugs in the bug tracking system.
There's new improvements included in this non-production release including class name resolution with "class" keyword and the DateTimeImmutable class. You can see the NEWS file for the complete list of changes. If you'd like to help test it out, you can download the preview release here (or here for Windows users).