When I want to remove a Cookie I try
unset($_COOKIE[\'hello\']);
I see in my cookie browser from firefox that the cookie still exists. How
If you want to delete the cookie completely from all your current domain then the following code will definitely help you.
unset($_COOKIE['hello']);
setcookie("hello", "", time() - 300,"/");
This code will delete the cookie variable completely from all your domain i.e; " / " - it denotes that cookie variable's value all set for all domain not just for current domain or path. time() - 300 denotes that it sets to a previous time so it will expire.
Thats how it's perfectly deleted.