How can I get a PHP cookie set on SUb.DOMAIN.COM so it works on SUB2.DOMAIN.com?

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It seems that a cookie set on one sub-domain is not readable on another sub-domain, even though FireFox shows the cookie as being there in Page Info when I go to the second subdomain.

I tried this:
setcookie("CookieName2011", $cookiecontents, time()+31536000, "/", ".domain.com");
but it still seems to only be readable on the sub-domain it was created within.