PHP 5 added more functionality to deal with object oriented code. Public/private functions, static functions, magic methods like __get, __set, etc.. Recently, php 5.3 added support for namespaces, late static binding, mysqlnd, and more. PHP 6 is supposed to add internationalization support.
Read the php news archive, for example here are the most important features announced for php 5.3: php.net/archive/2009.php#id2009-06-30-1