From the main PHP site today:

PHP 4.3.7RC1 has been released for testing. This is the first release candidate and should have a very low number of problems and/or bugs. Nevertheless, please download and test it as much as possible on real-life applications to uncover any remaining issues.

List of changes can be found in the NEWS file.

It looks like it was just a small number of errors that needed corrected, but that's just one step closer to a final of PHP 4.3.7 (and closer to the release of PHP 5 in the meantime).