On PHPMaster.com today there's a new tutorial showing you how to use the XPath functionality that's built into PHP's DOM functionality to query your XML.

In a recent article I discussed PHP’s implementation of the DOM and introduced various functions to pull data from and manipulate an XML structure. I also briefly mentioned XPath, but didn’t have much space to discuss it. In this article, we’ll look closer at XPath, how it functions, and how it is implemented in PHP. You’ll find that XPath can greatly reduce the amount of code you have to write to query and filter XML data, and will often yield better performance as well.

They start with some basic XPath queries to find a simple path and locating the record for a specific book. There's also an example of using XPath versus the "find" functions in the DOM functionality (like getElementsByTagName). There's also a bit close to the end about using functions in XPath and how you can pull back in PHP functionality and use native PHP functions in your XPath queries.