In this recent post to his site Greg Freeman share some things to check for when you think your PHP site (any kind, even something like WordPress) might have been compromised.

A friend of mine recently had their site compromised, they were running an older version of IP.Board that is vulnerable to a local file inclusion vulnerability. This post won't be about IP.Board or any specific php code, it will show you how to locate potential malicious php code hosted on your servers and how to fix it. Finally I will give a brief explanation on what attacker's are uploading to compromised sites.

Among the things he recommends are tips like:

• Check your Access Logs
• Finding Recently Modified PHP Files
• Finding obfuscated code
• Always search your writable upload directories for executable code
• Check .htaccess Files if you use Apache

You can find the descriptions for each of these (and some others to watch out for) in the full post.