This could be the sequel of this question: PHP Sessions across sub domains
I have a successful multi-domain session simply using this:
session_set_co
Did you see if the session is started before you try to destroy it?
if (!isset($_SESSION)) session_start();
if (isset($_COOKIE['auth_token'])) {
remove_token($_COOKIE['auth_token']);
setcookie("auth_token", "", time()-3600, "/", ".domain.com");
}
session_destroy();
This is working code from my environment. Hope it helps.