On the PHP.net site today they've announced the release of the latest Release Candidate in the PHP 5.4 series - PHP 5.4 RC1.

The PHP development team is proud to announce the first release candidate of PHP 5.4. PHP 5.4 includes new language features and removes several legacy (deprecated) behaviours. Windows binaries can be downloaded from the Windows QA site. [...] This is the first release candidate. No new features will be included before the final version of PHP 5.4.0. The release candidate phase is intended as a period of bug fixing prior to the stable release.

Major changes include class member access on instantiation, array to string conversion now throws a notice and bugs fixes/improvements to Core and other extensions. For a complete list, check out the NEWS file. Download this latest release