Case in point:
By default, Yahoo! Web Services return output in XML format. Some, but not all, of the Yahoo! Web Services can also return Serialized PHP. To get output in Serialized PHP format, use the output=php parameter in the request:
http://api.search.yahoo.com/ ImageSearchService/V1/ imageSearch?appid=YahooDemo &query=Madonna&results=1 &output=php
The RESTful way to do this is with an Accept Request header field.
I did point this out to Rasmus during a Web Services BOF at FOSS.in to which he said that it was easier to call it REST than call it HTTP. I don't buy this. A web developer will be much more familiar with the word HTTP (the thingy you see in the address bar of your browser and in the URI I've quoted above from Yahoo!'s own documentation) than with the word REST (an architectural style for distributed hypermedia systems described in a dissertation by Roy T. Fielding).
Now that that's out of my system, I really like what they are doing with PHP, especially this. I'm using Serialized PHP for representations as well in a web application I'm working on using Inertia with the following media type: application/x-serialized-php.