do this, have your variables you want to output in an array let's say $data, then pass that array to the view using the $this->set('data', $data); method, then create a view /General/SerializeJson.ctp. In that view file, put <?PHP echo json_encode($data); ?> after that you can use $this->render('/General/SerializeJson'); and it should output the json.
General code...
/Controllers/MyController.php
public class MyController extends AppController
{
public function ajaxAction()
{
$data = Array(
"name" => "Saad Imran",
"age" => 19
);
$this->set('data', $data);
$this->render('/General/SerializeJson/');
}
}
/Views/General/SerializeJson.ctp
<?PHP echo json_encode($data); ?>