无法使用namevaluepair将字符串发送到php文件

我正在尝试使用namevaluepair向php脚本发送字符串。但我无法在另一边收到它。这是我的密码

protected String doInBackground(String... args) {
  ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
  nameValuePairs.add(new BasicNameValuePair("Username",code ));
  Log.v("username", code);

  DefaultHttpClient httpclient = new DefaultHttpClient(new BasicHttpParams());
  HttpPost httppost = new HttpPost("http://192.168.42.21:8080/sellapp/menuitem.php");

  // Depends on your web service
  httppost.setHeader("Content-type", "application/json");

  InputStream inputStream = null;
  String result = null;


  try {
    httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
    HttpResponse response = httpclient.execute(httppost);
    HttpEntity entity = response.getEntity();

    inputStream = entity.getContent();
    // json is UTF-8 by default
    BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
    StringBuilder sb = new StringBuilder();

    String line = null;
    while ((line = reader.readLine()) != null)
    {
      sb.append(line + "\n");
    }
    result = sb.toString();


  } catch (Exception e) {
    // Oops
  }
  finally {
    try{if(inputStream != null)inputStream.close();}catch(Exception squish {}
  }
  return result;
}

当我直接给php赋值时，它就工作了。但通过名称/值对，它返回一个空数组作为结果

protected String doInBackground(String... args) {
  ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
  nameValuePairs.add(new BasicNameValuePair("Username",code ));
  Log.v("username", code);

  DefaultHttpClient httpclient = new DefaultHttpClient(new BasicHttpParams());
  HttpPost httppost = new HttpPost("http://192.168.42.21:8080/sellapp/menuitem.php");

  // Depends on your web service
  httppost.setHeader("Content-type", "application/json");

  InputStream inputStream = null;
  String result = null;


  try {
    httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
    HttpResponse response = httpclient.execute(httppost);
    HttpEntity entity = response.getEntity();

    inputStream = entity.getContent();
    // json is UTF-8 by default
    BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
    StringBuilder sb = new StringBuilder();

    String line = null;
    while ((line = reader.readLine()) != null)
    {
      sb.append(line + "\n");
    }
    result = sb.toString();


  } catch (Exception e) {
    // Oops
  }
  finally {
    try{if(inputStream != null)inputStream.close();}catch(Exception squish {}
  }
  return result;
}

---

请帮我找到答案。我试过相关的问题。但是没有起作用。

我仍然得到如下错误：V/myJSONs（438）：注意：未定义的索引：第9行的C:\xampp\htdocs\Sellapp\menuitem.php中的用户名
V/myJSONs（438）：{“结果”：[]}V/RenderScript（438）：应用程序请求CPU执行
protected String doInBackground(String... args) {
  ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
  nameValuePairs.add(new BasicNameValuePair("Username",code ));
  Log.v("username", code);

  DefaultHttpClient httpclient = new DefaultHttpClient(new BasicHttpParams());
  HttpPost httppost = new HttpPost("http://192.168.42.21:8080/sellapp/menuitem.php");

  // Depends on your web service
  httppost.setHeader("Content-type", "application/json");

  InputStream inputStream = null;
  String result = null;


  try {
    httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
    HttpResponse response = httpclient.execute(httppost);
    HttpEntity entity = response.getEntity();

    inputStream = entity.getContent();
    // json is UTF-8 by default
    BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
    StringBuilder sb = new StringBuilder();

    String line = null;
    while ((line = reader.readLine()) != null)
    {
      sb.append(line + "\n");
    }
    result = sb.toString();


  } catch (Exception e) {
    // Oops
  }
  finally {
    try{if(inputStream != null)inputStream.close();}catch(Exception squish {}
  }
  return result;
}

<?php


    $con = mysqli_connect(HOST,USER,PASS,DB);

    $cst_id = $_POST['Username']; // --------- not $_REQUEST['Username'];

    // $cst_id= 'cus02';

    $sql = "select  cust_code,segment_type,cust_name,cust_address,cust_payment_type,cust_credit_limit,cust_cr_balance from customer where cust_code='".$cst_id."' ";

    $res = mysqli_query($con,$sql);


    $result = array();
    while($row = mysqli_fetch_array($res)){
        array_push($result,
            ['cust_id'=>$row[0],
            'cust_seg'=>$row[1],
            'cust_name'=>$row[2],
            'cust_type'=>$row[3],
            'cust_ad'=>$row[4],
            'cust_cr'=>$row[5],
            'cust_bl'=>$row[6]
            ]);
    }

    //echo json_encode(array("result"=>$result));
    echo json_encode($result); 

    mysqli_close($con);


?>