On PHPMaster.com there's a new tutorial posted about using XML in PHP, an introduction to using the DOM functionality in PHP to work with your XML content.
PimpleXML allows you to quickly and easily work with XML documents, and in the majority of cases SimpleXML is sufficient. But if you're working with XML in any serious capacity, you'll eventually need a feature that isn't supported by SimpleXML, and that's where the PHP DOM (Document Object Model) comes in.
He starts with a brief introduction to XML and DTDs including an example of each (defining a sample book information he'll use in the rest of the tutorial). He helps you create a simple class that takes in the XML content, working with construction/destruction of the object and using it to find, add and delete a book by things like ISBN or genre.