HELP!!! - code injected into all my index.php files somehow
 

This has happened a few times now. Can someone tell me how this code keeps getting injected into my index.php files? It disables the forum. Seems easy enough to remove, but I've been hit with it like 3 times now, and I need to figure out why and how to stop this! Can anyone tell me where this is coming from and how to secure my server/forum?

This was at the top of my main index.php file...and just about all other index.php files. :confused:

PHP Code: