As far as I know, there aren't any PHP implementations for serializing / signing a transaction (this is part of my development roadmap for this proposal). But if you want a starting point, you can have a look at the JS implementation, a good starting point would be this area:

https://github.com/openhive-network/hivejs/blob/b60e2a65f57052ed341ab4bc405d60aaa27770c9/src/auth/index.js#L105