DevShed has started up a new tutorial series today with part one of their overview of the DOM extension that's included with PHP5:

Simply put, the DOM XML extension, as its name suggests, will permit you to work on XML documents by way of the DOM API. [...] In this article series I'll be discussing some of its most relevant methods and properties and accompanying all of these explanations with concise and instructive hands-on examples.

They look at using the DOM to create new documents, modify documents by adding additional nodes and converting over a SimpleXML document to be manipulated.