You are doing it wrong in $api->get
.
If you want to pass parameter to url with GET request then pass it in function get
as a SECOND parameter as array
$raw = $api->get('https://api.twitter.com/1.1/favorites/list.json',array('screen_name'=>'YSL'));
Here is the source code which says it self to pass $params
in second parameter as an Array :
function get($url, $params = array()) {
return $this->request('GET', $url, $params);
}
You can find it in oauthdamnit.php
line no 39