PHP Exploit Code in a GIF

That type of exploit is used against webservers, not against end-user PCs. It's a fairly trivial exploit that might allow a person to upload a piece of PHP code (PHP is a programming language used for writing web applications like forums, blogs and such) to run on a webserver, and perhaps take over some part or parts of that webserver given the right circumstances. But, it is not a file infector or trojan that effects Windows PCs.

Essentially, it's simply text in a specific format that is put inside a GIF image file, and it can only "run" if the machine its put on is running a webserver that has PHP installed on it, which is also configured to allow PHP code to be run in such a context. (Frankly, it's a low priority, non-issue exploit, that a basic "webserver 101" configuration should never allow to be exploited.)