There's a tutorial over on mpwebwizard.com that's been updated and talks about using a mirror to handle hits from a popular website (like digg or slashdot).

caching services such as coral content distribution network provide an alternative for when your site goes down, but why wait until it gets to that stage? why not just send all your hits from high-end websites to the coral cache version? well, there isn't a good reason not to, so i'll show you how to do it automatically.

He provides the simple script that looks for certain sites in the "HTTP_REFERER" value of incoming calls and pushes them off to the cached location with a header() redirect. The whole thing is only 12 lines, not counting the $sites array.

On the Bubble.ro website, there's an interesting tutorial that guides you through the creation of a histogram for any image (that PHP can handle) automatically with a little GD functionality.

The algorithm to calculate the histogram for an image is pretty straight forward if you know how to extract the RGB value of each pixel, by counting the number of pixels of each value (and then divide by the total number of pixels in order to get a normalized value).

They share the simple script that grabs the image's data, analyzes it, and, superimposed on one corner of it, places the histogram image. They have the full source and examples available for download.