Read carefully this announce from PHP.net and its links :

The PHP development team is proud to announce the immediate release of PHP 5.3.0. This release is a major improvement in the 5.X series, which includes a large number of new features and bug fixes.

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 5.3.0.

Further details about the PHP 5.3.0 release can be found in the release announcement, and the full list of changes are available in the ChangeLog.

This is a great news and a big step for PHP, that gets cleaner and speedier (for example, read this benchmark of ezPublish performance boost with PHP 5.3.0).