The main PHP.net site has the release announcement for the two latest versions of the language - PHP 5.3.3 and PHP 5.2.14.

The PHP development team would like to announce the immediate availability of PHP 5.3.3. This release focuses on improving the stability and security of the PHP 5.3.x branch with over 100 bug fixes, some of which are security related. All users are encouraged to upgrade to this release. [...] The PHP development team would like to announce the immediate availability of PHP 5.2.14. This release focuses on improving the stability of the PHP 5.2.x branch with over 60 bug fixes, some of which are security related.

The announcements list some of the major security enhancements and fixes in both new versions as well as a few new features like updates to the PCRE libraries and more.

Note: this PHP 5.2.14 release marks the end of active support for the PHP 5.2.x branch. It is encouraged that you upgrade to PHP 5.3 by following the steps in this migration guide.

As part of a project he was working up, Cory Rauch has created some statistics comparing the performance of the latest PHP 5 series release, PHP 5.2, with the previous version, PHP 5.1.

I can't say the results of the benchmarks are surprising, but it does give a good idea of where, speed-wise, they really improved things in this new release. There are some stats, though, that didn't make that much of a jump. Those seem to be ones dealing with objects and their handling, though - so there's not that much of a jump there anyway.

He tested all sorts of operations, including working with binary data, string and array functionality, looping (for, foreach, etc), and operators. For each statistic, he provides what the test is doing and what the results are. Testing was done with the PHPLens benchmark suite.