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.