使用AJAX调用特定的PHP函数

I'm trying to retrieve data from another server (testing locally) by calling a specific function within a PHP file. Currently nothing is being returned unless I manually call the function in the PHP file.

 $.ajax({
      type: 'POST',
      url: 'http://test.local/php/ajax/json.php',
      dataType: 'jsonp',
      data: {action: 'get_data'},
      success: function(data) {
        $json = $.parseJSON(data);
        [...]
      }
});

My PHP is as follows, and evidently $_POST is not set as the client application receives no data. Setting $_POST['action'] = 'get_data'; manually or calling get_data() in the PHP file returns all the data fine.

if (isset($_POST['action']) && !empty($_POST['action'])) {
  $action = $_POST['action'];
  switch ($action) {
    case 'get_data': get_data(); break;
  }
}

Why is $_POST['action'] not being set by the AJAX query? Any help is appreciated.