New versions of both generations of the PHP language have been released today - versions 5.2.1 and 4.4.5:

The PHP development team would like to announce the immediate availability of PHP 5.2.1 and availability of PHP 4.4.5. These releases are major stability and security enhancements of the 5.x and 4.4.x branches, and all users are strongly encouraged to upgrade to it as soon as possible. Further details about the PHP 5.2.1 release can be found in the release announcement for 5.2.1, the full list of changes is available in the ChangeLog for PHP 5. Details about the PHP 4.4.5 release can be found in the release announcement for 4.4.5, the full list of changes is available in the ChangeLog for PHP 4.

You can find what updates were made in each of the releases in their appropriate changelogs and from the main PHP.net website.