how to return json in an AJAX callback in D7?

How to return json?

In my js, I have...

var posturl = '/mymodule/my/callback';
$.post(posturl, {}, function(r) {
    console.log(r);
}, "json");

In my PHP...

function mymodule_page_my_callback() {

  $arr['apple'];
  $arr['banana'];
  
  drupal_json_output($arr);
// also tried: ajax_deliver($arr);
// also tried: print json_encode($arr);
// also had a look at the ajax example module but it does not use json.
  
}

In the firebug console I get a null response.

Thanks for your help.