HAXPLORER - any info?

My buddy just called me and let me know he found this running on his server. I have googled it and come up with much, but it appears to be a php script that gives the user pretty much total control of adding, editing, deleting - files, folders, etc. As well as giving them access to run any shell script they want. Does anyone know anything about this, and if they do - have any thoughts on how it may have gotten on the server and how to secure against it in the future? Just trying to help out a friend here. Thanks, Charles