This release focuses on addressing a regression in open_basedir implementation introduced in 5.2.15 in addition to fixing a crash inside PDO::pgsql on data retrieval when the server is down. All users who have upgraded to 5.2.15 and are utilizing open_basedir are strongly encouraged to upgrade to 5.2.16 or 5.3.4.

They also link to the PHP 5.3 migration guide to help make the upgrade to the world of PHP 5.3 simpler. If you want the full list of changes in this release, check out the Changelog.