On the PHP Security Blog today, this note has been posted, a notification that a critical vulnerability has finally been fixed - the unset() issue.
Because there are meanwhile a lot of rumours about this vulnerability in the underground and because the PHP 4.4.3 release announcement does not mention this critical hole at all I wrote up a little article about it, which you can read here.
The article (from Hardened PHP) describes the issue - a problem in the hash tables of the Zend Engine, specifically the zend_hash_del_key_or_index function. The logic contained inside the function can find the wrong "bucket" of information and remove it. He also includes PHP code examples that show the issue in action.
To be protected, it's recommended to update to the latest versions of PHP that have been released - 4.4.3 and 5.1.4.