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.
I just moved this thread in from the Webmaster Hardware forum to the PHP forum, a better home for it. For those experiencing problems with recent updates, please see the release notes in the links above.