Screaming Penguin - Source Does Matter

There are actually wuite a few potential PHP exploits, including a replicating "virus" type of a script, it was only a matter of time. A new script called PHP.pirus checks for files with .php and .htm extensions (apparently searches webservers?) and the implants itself into any of those files that are writeable. This appears to be the first self replicating PHP based "virus."

This is why a good sysadmin and knowing what you are doing is so important, virtually zero files should ever be writable by the default web server user. If this is the case, then this is a moot point, but if not, well, here we go,

Check the SARC storylink for more details.   PHP Virus: PHP.pirus