使用Ajax将JSON数据发送到PHP脚本

I have currently a problem with my code. I would like to send JSON data with Ajax to a PHP script but it doesn't work. What does work is that the PHP script can be called by the Ajax code but it can't put the code into the .txt file. I have tried several things but I can't get it working. (I am trying to set the users array in the .txt file)

jQuery code:

          var users = [];              

          $.ajax({
              type: "POST",
              url: hostURL + "sendto.php",
              dataType: 'json',
              data: { json: JSON.stringify(users) },
              success: function (data) {
                  alert(data);
              }
          });

PHP Code:

<?php
$json = $_POST['json'];
$data = json_decode($json);

$file = fopen('test.txt','w+');
fwrite($file, $data);
fclose($file);

echo 'Success?';
?>

You must know that in PHP json_decode generates an Array that you can't write into an text file.

So only remove the json_decode command.