The IBM developerWorks site has posted a tutorial (you'll need to log in) showing how to use native XML databases to speed up development time for your applications.

Native XML databases have grown in popularity along with XML, because data is stored as native XML, rather than through tables in a traditional database. Using a native XML database means that a change to the schema requires minimal changes to your code and no change to the database. PHP and Java developers can benefit greatly from using native XML databases

IT talks about how to connect to the database (in this case DB2), grab the XML data via a query and how to insert information back in via a web-based form. There's also an example showing how to make a "search" on the data and how to change the schema of the database on the fly as well.

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.