Here’s an interesting read from Jacob Santos on getting PHP to multi-task.

From time to time you hear about multithreading in PHP and how it is impossible. Yes, PHP does take the top-down approach to execution. It is possible to do multitasking by using Process Control Extension, but multitasking isn’t exactly the same thing as multithreading. 

Link: Multitasking in PHP - SantosJ Web Log - About my projects and what I’m currently doing