I remember back when I as learning PHP I had to write a directory "spider" function to list the contents of a directory. (Including sub-directories).
The approach I took involved reading the initial contents of the base directory into an array. The plan was then to loop through the array adding any sub-directories, (and their files), recursively.
A while back, I decided to use the Picasa system as a way of managing my photographs on my home PC and publishing the photos to the web.
I was planning on using the API for the system as provided by google, but I am limited to using PHP4 by my web host. (The PHP API classes rely on PHP5). So, I decided to create a PHP4 class for reading my Picasa web albums, ready to be dropped onto my website.
The class is basically a custom XML paser - quite a fun little project for a boring weekend!