Warum gibt meine GET-Funktion meiner API REST keinen Wert zurück?

ändern

$app->response->headers->set("Content-type","application/json");
$app->response->status(200);
$app->response->body(json_encode($users));

zu

header("HTTP/1.1 200");
header("Content-Type:application/json; charset=utf-8");
echo json_encode($users);