On the Perplexed Labs blog there's a recent post looking at how to fork processes in PHP with the help of the pcntl_fork function and the process management extension.

Let's say you want to take advantage of more than one core for a given process. Perhaps it performs many intensive computations and on a single core would take an hour to run. Since a PHP process is single threaded you won't optimally take advantage of the available multi-core resources you may have. Fortunately, via the Process Control (PCNTL) extension, PHP provides a way to fork new child processes.

He gives a quick snippet of code showing how to spawn off a few new processes, get their process IDs and watches a max number of children until one dies (then starts another).