Padraic Brady has started up a new series of posts to his blog on the topic of asynchronous processing with PHP.

Imagine a world where clients will give up on receiving responses from your application in mere seconds, where failed emails will give rise to complaints and lost business, where there exist tasks that must be performed regularly regardless of how many requests your application receives. This is not a fantasy world, it's reality. [...] Asynchronous processing is a method of performing tasks outside the loop of the current request. Basically, you offload the task to another process, leaving the process serving the request free to respond quickly and without delay.

He talks about some of the problems that asynchronous processing solves (like tasks that must be completed despite errors) and starts to outline a simple processing structure with child processes and task handling.