From the PHP website:
PHP 5.4.0 released!
The PHP development team is proud to announce the immediate availability of PHP 5.4.0. This release is a major leap forward in the 5.x series, which includes a large number of new features and bug fixes.
Some of the key new features include: [traits, [url=http://docs.php.net/manual/en/language.types.array.php]a shortened array syntax, [url=http://php.net/manual/en/features.commandline.webserver.php]a built-in webserver for testing purposes](http://us3.php.net/traits) 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.
Further details about the PHP 5.4.0 release can be found in the [release announcement, and the full list of changes are available in the [url=http://php.net/ChangeLog-5.php]ChangeLog](http://us.php.net/migration54).