Update May 2010: This is one of my most popular blog entries, so it seems worthwhile to modernize it a little. I've added an example of a generic REST helper that I've been using in a couple of places below the original do_post_request function in this entry. Enjoy!
I don't think we do a very good job of evangelizing some of the nice things that the PHP streams layer does in the PHP manual, or even in general. At least, every time I search for the code snippet that allows you to do an HTTP POST request, I don't find it in the manual and resort to reading the source. (You can find it if you search for "HTTP wrapper" in the online documentation, but that's not really what you think you're searching for when you're looking).
So, here's an example of how to send a POST request with straight up PHP, no cURL: