In your example, it’s fine as it is: it’s simple and works. The only things I’d suggest are:
make sure your API is sending the Content-Type header to tell the client to expect a JSON response:
header('Content-Type: application/json');
echo json_encode($response);
Other than that, an API is something that takes an input and provides an output. It’s possible to “over-engineer” things, in that you make things more complicated that need be.
If you wanted to go down the route of controllers and models, then read up on the MVC pattern and work out how your domain objects fit into it. Looking at the above example, I can see maybe a MathController with an add() action/method.
There are a few starting point projects for RESTful APIs on GitHub that are worth a look.