Cookies must be deleted with the same settings with which they were set. If the value argument is an empty string or FALSE, and all other arguments correspond to the previous setcookie call, the cookie with the specified name will be deleted from the remote client. This is internally achieved by setting the value to "remote" and the validity period to one year in the past.
From the official guide: http://php.net/manual/en/function.setcookie.php