UPDATE (2012): I’ve recently published a second edition of this tutorial that updates the original with Alfresco Share and CMIS.
I’ve written a new article (with sample files) that talks about how to extend Alfresco with your own content model and how to work with content that leverages that model via the Web Services API. All of the examples are written in Java but I do include one in PHP just for grins.
Most of the code is based on the Alfresco SDK Web Services sample code, but I’ve tweaked it here and there and I break it down into smaller chunks with commentary. I also think it is good to have one example to follow that takes you from designing the content model to implementing it to writing code that might leverage it.
More about the Alfresco Developer Series.