Questions about using a cookie under a domain name under level two domain _javascript tips

If we want to use a cookie under Www.jb51.net, just set a cookie in the file below this station.
But what if we want to implement cookies that are set in a first-level domain, and use it under a level two domain?

For example: We in the first level domain name is a website such as: Www.jb51.net, he below has a level two domain name like: bbs.jb51.net
We want to achieve in the first level domain name after login, keep this cookie to level two domain name, that is, achieve the effect of synchronous login,
So how do we set this cookie?? It's actually very simple.

Take just that domain name (www.jb51.net), we can set the Cookie:setcookie ("Fangbinbin", "Mengfei", Time () +3600, "/", "jb51.net");

Then we can use directly under this www.jb51.net and Bbs.jb51.net (Echo $_cookie[' Fangbinbin ';) to output the value of the COOKIE set.

This way to solve the problem of synchronous login!!