קרא על הפרמטר החמישי בsetcookie
http://il.php.net/manual/en/function.setcookie.php
קוד:
The domain that the cookie is available. To make the cookie available on all subdomains of example.com then you'd set it to '.example.com'.
PHP קוד:
setcookie("cookie", "cookie", time()+3600, "", ".YourDomain.com");