On the CSS-Tricks.com site today there's a quick tutorial on changing up the look and feel of your site based on an external source. More specifically, they give the example of updating the graphics of your site depending on the weather in your area via PHP and CSS.

Using a little magic and trickery (read: PHP and CSS), we can change the appearance of a website automatically based on the weather outside, in real time! In the example site we have created, the header graphic will change to one of four different styles based on Sunny, Rain, Snow, and Cloudy.

Their example makes a request to the Yahoo! weather data for a location and brings it in to PHP where the XML is parsed (via a regular expression) and the current conditions are parsed out. This condition is then passed out into the page as the class type on the header and, based on the CSS already defined, the correct image is pulled in as the background.