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 announced on PHP.net - the latest release candidate in the PHP 5.3 series has been made available for download - PHP 5.3.0RC1.

The PHP development team is proud to announce the availability of the first release candidate of PHP 5.3.0 (PHP 5.3.0RC1). This release marks the final phase in a major improvement in the 5.X series, which includes a large number of new features, bug fixes and security enhancements.

Key features of this new release include: namespaces, late static binding, better garbage collection, mysqlnd driver support, new bundled extensions (like phar, intl, fileinfo and sqlite3) as well as many, many bug fixes. You can find out more about all of the changes in this version, check out the NEWS file and can download the release candidate from the QA site here.