phpRiot.com: Using Callback Functions in PHP
A commonly used technique when programming is to use callbacks. A callback is a piece of code that is passed to a function that is later executed by that function. In this article I will show you how to define and call callbacks in PHP, as well as a practical example of how callbacks can be useful.
He starts with a normal script flow that has to wait until everything's done in order to handle the results. He then compares this to using a callback method that can be executed during each iteration of the script, reducing the amount of processing that has to be done after the fact. Code snippets are included of an example that pulls from a remote RSS feed and loops through the results.