The PHP development team announces the immediate availability of PHP 5.3.17. These releases fix over 20 bugs. All users of PHP are encouraged to upgrade to PHP 5.4.7, or at least 5.3.17.
PHP 5.3.x includes a large number of new features.
Some of the key new features include: namespaces, late static binding, closures, optional garbage collection for cyclic references, new extensions (like ext/phar, ext/intl and ext/fileinfo), over 140 bug fixes and much more.
For users upgrading from PHP 5.2 there is a migration guide available here, detailing the changes between those releases and PHP 5.3.x.
The full list of changes is available in the ChangeLog.