First,
$ctlr->$url[1]();
is invalid php.
It should look like
$ctlr->url[1];
and then, it really doesn't do anything.
I'd imagine you copied this from somewhere and are quite clueless as to what it does, and more importantly, how you would do it.
$ctlr = new $url[0]();
doesn't do you any favors, either.
Meanwhile - your error is screaming to your face exactly what it thinks is wrong.
So - in answer to your question: study php.