New on the Developer Tutorials blog today is this look at handling background processes from your PHP script:

You've checked and double checked the integrity of user input, and you’re doing some serious processing. There's only one problem: it's too slow. There's a simple solution: forking your processing script, and running the code as a background process asynchronously. It can email your user when it's done: they'll wait. In this tutorial, I'll show you how to get started with background processes in PHP.

Akash gives examples of the three keys to background processes - starting the script via an exec, talking to the process by passing additional parameters and including code to monitor the state of the background process via something like a MySQL "sessions" table that the script writes to.