To delete a cookie, you must issue it again and set a date in the past. It is important for all parameters in your new setcookie call except the timestamp to be identical to the parameters when the cookie was first issued; otherwise, the deletion will fail. Therefore, to delete the cookie created earlier, you would use the following:
setcookie('username', 'Hannah', time() - 2592000, '/');
As long as the time given is in the past, the cookie should be deleted. However, I have used a time of 2592000 seconds (one month) in the past in case the client computer’s date and time are not correctly set.