The PHP development team released 5.4.0, it is the next major milestone in the development of PHP. I’ve successfully tested it on my servers without a noticeable problem. First use this on a development machine, if you can wait for 5.4.1 before putting it on a production machine there might be some nasty bugs in it which haven’t been found.

Some of the key new features include (copied from the PHP website):
traits, a shortened array syntax, a built-in webserver for testing purposes and more. PHP 5.4.0 significantly improves performance, memory footprint and fixes over 100 bugs.

For users upgrading from PHP 5.3 there is a migration guide available here, detailing the changes between those releases and PHP 5.4.0.

For instructions on how to upgrade your PHP installation please read: Upgrading PHP.