Over the last few days I have been digging deep into the concepts of REST. I have been reading through my copy of RESTful Web Services. Out of curiosity I searched Google for a "php REST library" and it turned up the Official MySpace PHP REST Library.
He points out one interesting thing about it, though - it's like they started using an object-oriented design but build after making the classes with properties. No methods.
Even if they were to choose to go the PHP 5 way I would highly suggest that they do not use classes as containers for constants, there is no need to do this. It is not using proper OOP methodologies. Why create an object that just holds 2 - 4 constant variables, these would be better suited for define() calls in my opinion.