Mike Gauthier has created a PEAR-based implementation for the popular Akismet spam filtering system that he's added to the site:
I'd like to let you know about a new PHP 5 implementation of the Akismet REST API. The package, Services_Akismet is MIT licensed and distributed through the official PEAR channel at http://pear.php.net/package/Services_Akismet.
Hopefully this will make it even easier for third-party PHP projects to use Akismet spam tools. The code is well documented and should "just work" on any PHP 5.2.1+ system.
It's currently in its alpha stage and was derived from the miPHP Akismet class written by Bret Kuhns.