Some thoughts on the security aspect. I know theres a *nix function called "chroot", which lets you redefine the root of a process. I assume theres also someway to limit the cpu / memory usage of a process via similar means? What if the parent script forked a child that would just execute the user submitted code, but execute it only inside this chroot and under process restrictions? This way you don't have to worry about messing about with Safe or any such ops, and you can even give the user a small amount of control over his own file system, with the ability to create tmp files and such.