I recently read about http://php.net/pcntl and was woundering how good that functions works and if it would be smart to use multithreading in PHP since it isn\'t a core func
This is not thread control, this is process control. The library for the threads is pthreads (POSIX threads) and it's not included in PHP, so there are no multi-threading functions in PHP.
As of multiprocessing, you cannot use that in mod_php, as that would be a giant security hole (spawned process would have all the web-server's privileges).