Lithium Hosting uses SuPHP to secure PHP. suPHP is a tool for executing PHP scripts with the permissions of their owners. It consists of an Apache module (mod_suphp) and a setuid root binary (suphp) that is called by the Apache module to change the uid of the process executing the PHP interpreter.
If you are getting 500 Server Errors on any of your PHP scripts, please be sure your permissions are set to 644. This means they are world readable. Under no circumstances should any script be set higher than 644 or folder be set to 777.