Multithreading in PHP
The pcntl package works quite fine - it just uses the according unix functions. The only shortage is that you can't use them if php is invoked from a web server context. i.e. you can use it in shell scripts, but not on web pages - at least not without using a hack like calling a forking script with exec or similar.
[edit]I just found a page explaining why mod_php cannot fork. Basically it's a security issue.[/edit]
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).
The only possible way to have php code executing in multiple threads is to run php as a module of a threaded web server, which is useless because threads are fully isolated and your code has no control over them. As far as i know, pcntl only manages processes, not threads.