Since the introduction of PHP5, a range of XML handling functions have been added to PHP core. Why is this important I hear you ask? Well it makes life easier if you work with XML!
Below is a small script that uses SimpleXML loads and parses an RSS feed (which is XML) from this site. The feed is generated automatically by WordPress. It then displays some of the feed meta data and a list of posts contained in the feed.
Full SimpleXML documentation can be found here.
And you can see the code in action here.