Today the latest version in the 5.2 released 5.2.14 focus on improving the stability with over 60 bugs fixes, including security fixes. This release marks the end of the active support for PHP 5.2. Following this release the PHP 5.2 series will receive no further active bug maintenance. Security fixes for PHP 5.2 might be published on a case by cases basis. All users of PHP 5.2 are encouraged to upgrade to PHP 5.3.
On the 5.3 branch, the 5.3.3 released with over 100 bug fixes, some of which are security related. All users are encouraged to upgrade to this release. There is a backwards incompatible change between 5.3.2 and 5.3.3 in namespaces : Methods with the same name as the last element of a namespaced class name will no longer be treated as constructor. This change doesn’t affect non-namespaced classes.
There is no impact on migration from 5.2.x because namespaces were only introduced in PHP 5.3.
To prepare for upgrading to PHP 5.3, now that PHP 5.2’s support ended, a migration guide available on http://php.net/migration53, details the changes between PHP 5.2 and PHP 5.3.