On the Re-Cycled Air blog Jack Slingerland has posted another in his "Dark Arts" series looking at some of the lesser used PHP features. This time he focuses in on daemonizing a process by forking it off into the background.
One of the many things you don't often do with PHP (actually, I'm not sure you do this much with any language) is daemonize a process. A daemon is program that runs in the background (read more here). On Unix systems, processes are usually created by forking the init process and then manipulating the process to your liking. To create a daemon though, you need to get the init process to adopt your process. To do that, as soon as you fork the parent process, you kill the parent. Since you child process is parent-less, the init process generally adopts it. Once that happens, your process has been daemonized.
He uses the pcntl_fork function to spawn off the child process, detach it from a terminal window, create a ".pid" file so the system knows about it and then, of course, have the child script do something.