What to return in controller

Hello.
I would like to ask, what to return in controller if I am expecting XMLHttpRequest but request is not XMLHttpRequest.
I have something like this:

Code:

public function show_post($id)
{
 if (!$this->input->is_ajax_request())
   return false;
 ...
 $data = $this->some_model->post($id);
 return json_encode($data);
}

Is it good to return false or return other value?
Thank you.