Found two excellent articles today on Threading Vs. Forking, and using the forking extension in PHP (pcntl).
Suffice to say, being a Linux user, I prefer using forking rather than threading. I have had experience with threading in Java. I guess I just enjoy the safety net that forking gives you. I also prefer that fact that you have to have a defined protocol between two processes to achieve IPC (inter process communication). Whereas in threading each thread can access shared memory directly.