Creating a custom filter with CallbackFilterIterator

- The CallbackFilterIterator class allows you to create…a custom filter using a callback function.…It requires a minimum of PHP 5.4.…If you're using PHP 5.3, the alternative…is to extend the FilterIterator class…which is described later in this chapter.…I've got open callback_filter.php.…It contains the simplexml that was used…earlier in this chapter to extract data…from courses.xml in the common/data directory.…

The only difference is that instead of getting the duration,…this time we're getting the course level.…The first thing we need to do is to convert…the simplexml element into a simplexml iterator.…We do that by adding a second argument…to the simplexml_load_file.…So that's on line two and it's a string SimpleXMLIterator.…Forgetting to convert simplexml elements into iterators…when you want to use them with other iterators…is a common problem, so don't forget to do that.…

So, now we can chain this with the CallbackFilterIterator.…I think I'm going to need quite a lot of space…so I'm going to expand my editing pane.…