The Filter Iterator is probably my second favorite iterator, next to Directory Iterator. There are many great use cases for the Filter Iterator, and when you do filter the original data is left untouched. A Filter Iterator is really simple to use, create a class that extends FilterIterator and adjust the accept method to meet your criteria. This is great and all, but having the ability to create filter iterators on the fly, ones that won't be used application wide, without having to create a class is even better.
He includes a bit of code defining a FilterCallbackIterator class with a "callback" parameter passed into the constructor (in his case, a closure). Also included is some sample code of it in use - handling an array (well, ArrayIterator) with a simple true/false check on the current array value. You can find out more about this functionality in the PHP manual.