How to achieve session management purpose like google in PHP?

[ordix]

I mean you can login both \[code\]https://mail.google.com/\[/code\] and \[code\]https://mail.google.com/a/company.com\[/code\] at the same time.The projects I've attended so far haven't involved such kind of logic,how can these two url under the same domain use different \[code\]$_SESSION\[/code\]?