Selfmade Actor / io Module: Best way to send commands to it?

Add-ons Bindings
2

Why not? Use a Design Pattern: Proxy Item Switch to trigger a rule. Use the sendHttpGetRequest Action to send the command and update your PHP script on the RPi to return success or failure in the body of the return.

rule "RPi Actuator"
when
    Item ActuatorProxy received command
then
    val url = if(receivedCommand == ON) onURL else offURL
    val result = sendHttpGetRequest(url)
    if(result != "success") ActuatorProxy.postUpdate(if(receivedCommand == ON) OFF else ON)
end

I just typed the above in, there are likely typos.

Personally, I would probably use MQTT to send the command and an ack.