An extremely serious security flaw has been discovered in PHP, requiring that all affected servers be updated as a matter of urgency.
The flaw allows a remote webserver running an affected version of PHP to be crashed using nothing more than a URL request.
If you are running a 64 bit version of PHP you are unaffected, but if you are running in 32 bit mode, or you are not sure,
now would be a good time to drop everything and make sure that your server is not vulnerable,
by installing the latest version of PHP either from php.net, or from your own webserver vendor.
Zend Server has a hotfix available already.
Zend Server hotfix
Due to the massive impact of the flaw and the trivial way in which it can be exploited, news of this bug will spread rapidly so speed is of the essence in getting your server patched.
PHP script to test vulnerability
Bug report detailing the flaw