User::resolveUserId

static function resolveUserId($user)
{
	if ($user instanceof CurrentUser)
	{
		return $user->getId();
	}
	if($user instanceof User)
	{
		return (int)$user->getId();
	}
	if ($user instanceof CUser)
	{
		return (int)$user->getId();
	}
	elseif(is_numeric($user) && (int)$user > 0)
	{
		return (int)$user;
	}

	return null;
}