Problem with saving Json data in PhP?

Question

PHP

Sending data as json string to PHP server. The server sees them and creates a file for them, but does not save them, the server also gives an answer. Since I am a complete zero in PHP, I would like to get help, your help. So why doesn't the server save the data?
Here is the processing code:

<?php
  $r = $_POST['req'];

  $fn = fopen('reqest.txt','a+');
  fwrite($fn, $r);
  fclose($fn);

$arr = array('status'=>ok,'code'=>2,'monitor_frequency'=>2);
// $test = array();
echo json_encode($arr);

?>

Here is the send code from the Android app:

static  String SendJsonViaPost(String url, String json) {

            HttpClient client = new DefaultHttpClient();
            HttpPost post = new HttpPost(url);
            try {
               // StringEntity stringEntity = new StringEntity(json, "UTF-8");
                //stringEntity.setContentType("application/json");
                //post.setEntity(stringEntity);
                List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
                nameValuePairs.add(new BasicNameValuePair("uid",m_szUniqueID));
                nameValuePairs.add(new BasicNameValuePair("data",json));
                post.setEntity(new UrlEncodedFormEntity(nameValuePairs,"UTF-8"));
                HttpResponse response = client.execute(post);
                BufferedReader rd = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
                String line = "";
                while ((line = rd.readLine()) != null) {
                    responseJson += line;
                }

            } catch (IOException e) {
                e.printStackTrace();
            }
            return responseJson;}

Answer 1

Do it like this

<?php
  $r = $_POST['req'];
  var_dump($r);

and tell me what it is..

Answer 2

file_put_contents('file.json', $json);

Answer 3

You pass 2 variables from Android to the server to the post "uid" and "data". And on the server you are trying to catch the "req" variable. inconsistency