Site got defaced (hacked with the web page modified). Generally done for fun, political reasons and by script kiddies.
Affecting
Any vulnerable PHP based website. Outdated software or compromised passwords can act as an infection vector.
Cleanup
When this happens the only approach is to restore the compromised index.php files from a backup. Also you can sign up with us and let our team help you restoring your sitea and removing any remaining malware for you.