Android HttpURLConnection Post Data，PHP App Server无法接收数据

Question

我正在使用HttpURLConnection将JSONObject数据发布到我的PHP后端。但是，我的后端PHP无法接收任何数据并回显“一些问题”。我需要从前端发布数据，从MySQL数据库中提取数据。以下是我的Android代码和PHP代码。

Android代码。

 try {
                URL urls = new URL(url);
                HttpURLConnection conn = (HttpURLConnection) urls.openConnection();
                conn.setDoOutput(true);
                conn.setDoInput(true);
                conn.setRequestProperty("Content-Type", "application/json");
                conn.setRequestProperty("Accept", "application/json");
                conn.setRequestMethod("POST");

                JSONObject deviceToken = new JSONObject();
                deviceToken.put("RegistrationID", getRegIDs);

                OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
                wr.write(deviceToken.toString());
                wr.flush();

                StringBuilder sb = new StringBuilder();
                int HttpResult = conn.getResponseCode();
                if(HttpResult == HttpURLConnection.HTTP_OK){
                    BufferedReader br = new BufferedReader(new InputStreamReader(conn.getInputStream(), "utf-8"));
                    String line = null;
                    while ((line = br.readLine()) != null){
                        sb.append(line + "\n");
                    }
                    DataString = sb.toString();
                    br.close();
                }

            } catch (MalformedURLException e) {
                Log.e(TAG, "MalformedURLException: " + e.getMessage());
            } catch (ProtocolException e) {
                Log.e(TAG, "ProtocolException: " + e.getMessage());
            } catch (IOException e) {
                Log.e(TAG, "IOException: " + e.getMessage());
            } catch (Exception e) {
                Log.e(TAG, "Exception: " + e.getMessage());
            }

PHP代码

<?php
    require_once('config.php'); 

    header("Access-Control-Allow-Origin: *");
    header("Access-Control-Allow-Credentials: true");
    header("Access-Control-Allow-Methods: POST, GET, OPTIONS");
    header("Access-Control-Allow-Headers: Content-Type, Authorization, X-Requested-With");
    header('P3P: CP="CAO PSA OUR"'); 
    header("Content-Type: application/json; charset=utf-8");

    $DeviceToken = $_POST['RegistrationID'];

    $ReceiverDataSQL = "SELECT * FROM xpouser WHERE (device_token = '$DeviceToken')";
    $ReceiverDataResult = mysqli_query($conn, $ReceiverDataSQL);
    $messages = mysqli_fetch_assoc($ReceiverDataResult);
    $ReceiverID = $messages['OID'];

    if(!empty($ReceiverID)){
        $channel_query = "SELECT * FROM xpochannel WHERE ChannelName LIKE '%$ReceiverID%'";
        $result = mysqli_query($conn, $channel_query);
        $msg = array();
        $string = null;

        while($messages = mysqli_fetch_assoc($result)){
            array_push($msg, $messages);
        }

        if (is_array($msg) || is_object($msg)){
            foreach($msg as $message){
                $OID = $message['OID'];
                $lastText_query = "SELECT * FROM xpomessage WHERE Recipient = '$OID'  ORDER BY OID DESC LIMIT 1";
                $lastText_result = mysqli_query($conn,  $lastText_query);
                $row_lastText = mysqli_fetch_assoc($lastText_result);
                $last_text = $row_lastText['Message'];

                $array = array(
                    'ChannelName' => $message['ChannelName'],
                    'LastText' => $last_text);  
                $obj = json_encode($array);
                $string .= $obj;
            }
        $returnObj = "[" .$string. "]";
        $new_str = str_replace('}{', '},{ ', $returnObj);

        echo $new_str;
    }
    }else{
        echo "Some Problem";
    }
?>