private static function destroy_cookie($type=null) {
	if (is_null($type)) {
		foreach (self::$type_durations as $type => $null) {
			self::destroy_cookie($type);
		}
		return;
	}
	
	if (self::cookie_exists($type) == false) {
		return;
	}
	
	self::update_cookie_expiration($type, $expire_now=true);
	
	$cookie_name = self::get_cookie_name($type);
	unset($_COOKIE[$cookie_name]);
}