PHP backdoors repository is a collection of obfuscated and deobfuscated PHP backdoors. (For educational or testing purposes only, obviously.) These provide a great insight into what kind of functionality the attackers are looking for when they exploit your application. Most of these rotate around file system operations, executing commands, and sending emails.
One of the things from those files that I haven’t seen before is FOPO – Free Online PHP Obfuscator tool.