Query prepared in web server with PHP JSON for Android

I have a small problem. Thanks to another partner here is how to prepare inserts prepared in the databases, but with that same model I do not know how to recover a prepared query and recover it in Android.

Problem 1 the query: Generally when implementing in the Response.Listener class it allowed me to recover a JSONObject and not here.

//Cargamos los anuncios
public void iniciarSesionAnuncios(){




    String url="https://gatos.com/anuncios.php?";
    stringRequest = new StringRequest(Request.Method.POST, url, new Response.Listener<String>() {
        @Override
        public void onResponse(String response) {

        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {

        }
    }){
        @Override
        protected Map<String, String> getParams() throws AuthFailureError {

            Map<String,String> parametro = new HashMap<>();

            String gatoID="12";
            parametro.put("gatoID",gatoID);


            return parametro;
        }
    };


}

Problem 2: How do I create a php json object that can be recovered with a ready query?