chrislph is correct, there is no good documentation for the current PHP API. You basically need to read the source code the figure out which endpoints exist.
We are working on Pi-hole v6 quite hard, which will have a new and extensive API. The API contains the documentation within itself and we added tests that will fail if an endpoint exists without documentation.
To get an idea what will be possible, have a look here: https://ftl.pi-hole.net/new/http/docs/#overview and especially: https://ftl.pi-hole.net/new/http/docs/#get-/endpoints
(API is still a moving target - things can an will change probably still a lot)