Kevin Schroeder's Blog: You want to do WHAT with PHP? Chapter 5
If you are doing any data processing whatsoever you are using arrays. And most likely you are doing database queries, iterating over the results and doing your algorithm-ing. But what if you have additional functionality that you need to have integrated with your data. You could go the traditional route and copy and paste half your application around or you could build, what we like to call structured applications. SPL allows you to do that.
In the excerpt he mentions the methods that come built in to Iterators like rewind, current and valid. There's also some sample code showing a custom iterator that works with some user data to output the information set in the constructor. You can find out more about this chapter and the rest in Kevin's book.