JSON API显示正确的结果，但在使用$ .getJSON和回调的窗口小部件请求时不会返回它们

I have created a JSON API using PHP for a desktop widget app I am developing, which has has the structure:

{"a":"b"}

The client (in other words the widget) requests the API using $.getJSON and requests the URL

"http://mylink.php?callback=?"

but no results are returned (whereas when I type the same URL directly in the browser, the results are there and correct). I also tried to request another JSON API (not created by me) and it works fine, it returns results. So I am almost sure that the problem is in the way I create my API.

The code for creating the JSON API in the server's side is the following:

<?php
  ..............
  $results = array();
  $results['a'] = 'b';
  $encoded = json_encode($results);
  $encoded_callback = '?' . '(' . $encoded . ')';
  echo $encoded_callback;
  return $encoded_callback;
?>

The javascript code only contains

$getJSON(url, output, function(...){
      ......
});

up to now and works fine with another API. I cannot figure out what I am doing wrong, either with the creation of the API from the server or with the URL request from the client. Do you have any ideas?