Maybe I'll write up something a little more formal in the future. For now, I just want to publish this in case it's useful to someone.
Les Orchard posted a blurb that indicated that he was looking for a PHP class to perform HTTP requests with conditional GET support. Well, a while ago I was looking for that, too. Because I was working on a replacement for Magpie RSS (see below), I decided to use Snoopy as my HTTP client. I then wrote a brief extension, Leash, to provide a cache-enabled front end to Snoopy. Leash automatically caches the HTTP results, the time of the request, and the Last Modified and Etag HTTP headers. When you request a page you've previously requested, Leash first checks to see if the cached copy is older than the maximum cache age you've specified (or the default of 1 hour), and if the cache is too old, Leash performs a conditional GET. The latest version of Leash (which I bundle with Snoopy) is in my Subversion repository.
Also in that repository is my replacement for Magpie RSS. I always liked Magpie, but it didn't quite work for me and I also wanted an OPML parser. So I wrote one. Actually, first I wrote a generic PHP XML parser. Then I wrote the OPML parser and Feed parser.
Sorry, but I currently don't have time for documentation. Or support. That probably makes this of very limited utility to all but the most daring. If you're a PHP junkie, you'll probably be able to peruse the code and get the gist. And here's an example of how I'm using it to help manage my podcasts.