if ($user) {
  try {
 $friendsapp = $facebook->api('/me/friends?fields=installed');
 } catch (FacebookApiException $e) {
    echo '<pre>'.htmlspecialchars(print_r($e, true)).'</pre>';
    $user = null;
 }
}

SELECT uid FROM user WHERE is_app_user=1 AND uid IN (SELECT uid2 FROM friend WHERE uid1 = me())