How to convert object to JSON in symfony2

I am using this:

    $users = $em->getRepository('UserBundle:User')->getallUsers($search);
    $response = new Response(json_encode($users));
    $response->headers->set('Content-Type', 'application/json');
    return $response;

Users are multiple entities not single result.

But I am getting this:

[{},{},{},{},{},{}]

I want something like:

[ { label: $user.getName(), value: $user.getId() }, ... ]

How can i do that?

EDIT: I also tried json_encode($users->toArray()) then I get this error:

Call to a member function toArray() on a non-object