To make use of multiple cores for some rather long processing operations I needed a way to fork multiple workers from a single PHP script multiple times lately. So I created a small project on github which implements this in a way, so that it should reusable by anybody. This is far from being rocket science, but still might be useful to someone out there.
He uses the ShellJobProvider (extended from JobProvider) to handle the creation of the tasks. His simple example just spawns off a few echo statements and pushes the results to some files. You can get these libraries from hos github account. You'll need PHP 5.3 and the PCTNL extension installed to get it working.