API. Transport solutions in the internal network

Question

PHP

The young project migrated to work on the principle of SOA .
Accordingly, all project services communicate with the API, as with a database layer.
The question is the correct selection of the transport method for the client API between internal services.
In particular, the speed of the transport itself and the speed of the client API code (php) are considered as a priority.
At this stage, the API client is using the Guzzle library as a temporary workaround, but it has been found to have a number of performance issues, and the library itself seems too monstrous.
In general, a number of ideas for improving transport were considered. For example, one of the exotic - GearmanWorker on HttpMessage serialization. That is, there is an opportunity to improve the API Service for receiving motley types of messages.
The question actually comes up next - what libraries and complex solutions for improving transport between services will the respected community recommend? Here I would like to consider both best-practice and "and here we are."
Thank you for your attention.

Answer 1

JSON-RPC (xml-rpc) is quite suitable for these purposes.
There are a lot of client and server libraries for many languages.