Spawning a background process

Allo, Basically, my question is.. how do I do this? I am actually (for various .. and probably stupid... reasons) trying to spawn a php process to run in the background. This process I know will take a couple of hours to run (mailing list stuff). I have been playing around with system to try to get it to run in the background but I can't seem to work out how to do it. Any pointers would be a great help Thanks Kevin