How to get the domain of a specific cookie?

I don't think the domain is available when reading cookies, this is limited by the browser. A solution would be to remove the old cookie and change it to the new domain.

E.g.

$value = $_COOKIE['TestCookie'];
setcookie("TestCookie", "", time() - 3600, "www.example.com");
setcookie("TestCookie", $value, time + (60 * 60 * 24 * 30), ".example.com");