I've just released a feature update for phpFlickr to support Flickr's new Authentication API. I included documentation on the new method in the readme file. I also included a couple of simple scripts (including a callback script) to help you use the authentication as painlessly as possible. I'd suggest using this new scheme as soon as possible as Flickr has promised to deactivate the old authentication method at some point, but haven't given a specific date or even a general date to indicate how long you'll have to switch over. If you're not using authentication, you probably won't need to change much at all.