如何使用Volley库在PHP站点服务器上接收JSONObject？

I try send and receive JSONObject on PHP site server. I use below sample code.

GlobalConfig config = new GlobalConfig();
JSONObject json = config.getConfig();

JSONObject data = new JSONObject();

try {

    data.put("ID", json.getString("ID"));
    data.put("session", json.getString("session"));

} catch (JSONException e) {
    e.printStackTrace();
}


RequestQueue queue = Volley.newRequestQueue(this);

JsonObjectRequest postRequest = new JsonObjectRequest(Request.Method.POST, PROT + "://" + REMOTE + "/" + DIR + "/test.php", data,
        new Response.Listener<JSONObject>()
        {
            @Override
            public void onResponse(JSONObject response) {
                // response

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

            }
        }
);

Below is my PHP script:

<?php

  $json = json_decode($_POST['json']);
  $date = date("Y-m-d H:i:s");

  $obj = new stdClass();
  $obj->id  = $json->ID;
  $obj->session = $json->session;

$myfile = fopen("newfile.txt", "w") or die("Unable to open file!");
$txt = "login: ".$obj->id.", ".session: ".$obj->session;
fwrite($myfile, $txt);
fclose($myfile);
?>

When I run my app, the file is created but only with the words login and session without variables. What is wrong?