Evert Pot has posted his latest update for the PHP-RPC spec for PHP - PHP-RPC update 4 - hopefully the last:

Unless somebody has some great feedback with stuff I overlooked - it might need some clarification and better writing here and there, but I think the general idea is there.

The api for the server class currently works the exact same as before, but support for multi-calls has been added. I also added a client class, which is helpful when you surpass the prototyping phase and you need a more decent way to interact with the service.

Details on the proposal are in the post including code for an example usage (and a multi-call use) and the official version of the latest proposal, version 0.4. This is broken out into the two sides of the proposed method - the request and the server - and how they would handle the interaction.