decode json object sent form android app to php server

I am sending json object from android as such:

//Create JSONObject here
                    JSONObject json = new JSONObject();
                    json.put("key", String.valueOf(args[0]));

                    String postData=json.toString();

                    // Send POST output.
                    printout = new DataOutputStream(urlConn.getOutputStream ());
                    printout.writeUTF(URLEncoder.encode(postData,"UTF-8"));
                    Log.i("NOTIFICATION", "Data Sent");
                    printout.flush ();
                    printout.close ();

When it is sent to the server it looks like the following code snippet. ???%7B%22key%22%3A%22value%22%7D I should add the first ??? are in a diamond each. When I decode the whole json object I get null. In the php server I have

$somevar=json_decode(json, true);

which returns null. Can someone point me on how to retrieve the json value? Thanks so much:)