On phpRiot.com there's a new tutorial that will walk you through the creation of a WebDAV file system based on PHP and the SabreDAV library.

We are going to create a simple web drive that stores its files in a directory on the web server. By the end you will be able to effectively map this directory to your local computer and update files using your native operating system. Put simply, WebDAV (Web-based Distributed Authoring and Versioning) is an extension of HTTP that allows you to manage files on a remote web server. It allows you to think of files and directories on a web site as though they are local files on your computer.

Over the course of the next few pages they introduce DAV, set up your webserver to handle the requests and creates a sample script you can have set up quickly and get working. They also mention some of the additional things like locking, authentication, and working with plugins.