There is one, it is called http or https. I agree that more direct interfaces would be nice, but there are currently methods of controlling and passing data to a PHP server from any language you can open a port in.
It takes a lot more that just http. I'd need to know the URL schema if you're restful, or the method names if you're SOAPy. Then there's the format of the data (SOAP, JSON, XML ..) and the expected data fields in the data sent and received, etc.
From the source, it looks like they're building abstracted PHP libraries, too. And it's an event/hook architecture, so you can pop this protocol out and put another one in, theoretically.
I don't know if this is a placeholder protocol while they build the software, or if this is what they plan on using and evolving.