Need help! about virtural directory

How to map a virtual directory into a real path?
(website) /test
--> (Real path) ???

that works like server.mappath("/test") in asp,
works under any web-server, typically IIS and Apache.
If there is, please tell me.

Note:
maybe the virtual path have no relationship with its virtual root.
Such as:
Virtual    Real path
/           /etc/website
/test       /etc/testpath