php variable not recognised.

I have the following php script where the user can rate a player from his Android phone and post that data in my server.

So:

<?php
  session_start();
  require "init.php";
  header('Content-type: application/json');

  error_reporting(E_ALL); 
  ini_set("display_errors", 1);

  $id = isset($_POST['player_id']);
  $user_id = isset($_POST['User_Id']);
  $best_player = isset($_POST['player']);
  $rate = isset($_POST['rating']);

  if($id && $user_id && $best_player && $rate){

    $sql_query = "insert into rating_players_table values('$id','$best_player','$rate','$user_id');";

    if(mysqli_query($con,$sql_query)){

        $don = array('result' =>"success","message"=>"Επιτυχής πρόσθεση παίχτη");
    }       
   }else if(!$best_player){

        $don = array('result' =>"fail","message"=>"Insert player name");

    }else if(!$rate){

        $don = array('result' =>"fail","message"=>"Rate player");

    }
 echo json_encode($don);

?>

However I get a message saying the $don variable inside the echo is not recognised.

The Android code that sends data is:

private void ratingPlayer(final String player, final int rating,final String UserId) {

    String tag_string_req = "req_register";

    StringRequest strReq = new StringRequest(Request.Method.POST,
            URL.URL_BEST_PLAYERS, new Response.Listener<String>() {
        @Override
        public void onResponse(String response) {
            Log.d("Response", "Best player response: " + response.toString());

            try {
                JSONObject jsonObject = new JSONObject(response);
                if (jsonObject.getString("result").equals("success")) {
                    Toast.makeText(getApplicationContext(), jsonObject.getString("message"), Toast.LENGTH_LONG).show();
                } else if (jsonObject.getString("result").equals("fail")) {
                    Toast.makeText(getApplicationContext(), jsonObject.getString("message"), Toast.LENGTH_LONG).show();
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }, new Response.ErrorListener() {

        @Override
        public void onErrorResponse(VolleyError error) {
            Log.e("Error", "Registration Error: " + error.getMessage());
            Toast.makeText(getApplicationContext(),
                    error.getMessage(), Toast.LENGTH_LONG).show();

        }
    }) {
        @Override
        protected Map<String, String> getParams() {
            // Posting params to register urls
            Map<String, String> params = new HashMap<String, String>();
            params.put("player_id", "");
            params.put("User_Id", UserId);
            params.put("player", player);
            params.put("rating", String.valueOf(rating));
            return params;
        }
    };
      // Adding request to request queue
      AppController.getInstance().addToRequestQueue(strReq, tag_string_req);
   }

where

final String player, final int rating,final String UserId

the values I put in my edit texts fields in the Android phone.

What could be wrong?

Thanks,

Theo.