php jsoncallback undefined

Question

最初一切正常，直到IT人员更换服务器 我用ajax调用php文件从数据库中获取数据。现在我得到一个错误 PHP注意：未定义的索引：jsoncallback

    header('Content-Type: application/json');
    $conn = mysqli_connect($servername, $username, $password, $dbname);
      // Check connection
      if (!$conn) {
        die("Connection failed: " . mysqli_connect_error());
      }


    $sql = "SELECT * FROM users";
    $result = mysqli_query($conn, $sql);
      $records = array();
      if (mysqli_num_rows($result) > 0) {
        while($row = mysqli_fetch_assoc($result)) {
            $records[] = $row;  
        }
      }

      mysqli_close($conn);



    echo $_GET['jsoncallback'] . '(' . json_encode($records) . ');';

====


      $.ajax({
                    url: 'http://test.php',
                    dataType: 'jsonp',
                    jsonp: 'jsoncallback',
                    async: false,
                    timeout: 5000,
                    success: function(data, status)
                    {
                    },
  error: function(){}
  });