如何获取好友/关注 ID

>我写了这段代码：

$twitterObj->setToken($_SESSION['ot'], $_SESSION['ots']);
$twitterFriends=$twitterObj->get_friendsIds(array('screen_name' => $_SESSION['username']));
$twitterFriends->response;
echo $twitterFriends->responseText;
foreach($twitterFriends as $friend) { echo $friend->id; }

但我明白这个：

{"errors":[{"code":37,"message":"Not authorized to use this endpoint"}]}

为什么？如何获取好友/关注ID？