詳細

ログイン Cookie のドメインを明示するために設定します。例: thissubdomain.mydomain.org、またはサブドメインすべての場合は .mydomain.org

By default this isn't set, so php's setcookie function will just set a cookie on the current domain. This is a bad thing if e.g. your site is available from both www.example.com and example.com, since users that log into the former will be logged out as soon as they visit the latter.