Just use the 1.1 in the URL. That will do
http://api.twitter.com/1.1/users/lookup.json
-------^
Do like this...
$connection = new TwitterOAuth(CONSUMER_KEY, CONSUMER_SECRET, OAUTH_TOKEN,OAUTH_SECRET);
$account = $connection->get('account/verify_credentials');
$status = $connection->post('statuses/update', array('status' => 'Text of status here', 'in_reply_to_status_id' => 123456));
$status = $connection->delete('statuses/destroy/12345');