Cookies

Currently having problems deleting cookies set using PHP.

I have a need to set them on one server and to be read by scripts in another server. To do so, I need to apply the top level domain when I set the cookies, otherwise I can't read them when the visitor moves subdomains. But for some reason, I cannot delete the cookie from the other server, even if the domain is specified in exactly the same way.

Perhaps its a security feature and therefore just not possible to do?