default domain name

Hello,

I need your help on something, i am developing an application, and there
are couple of modules where i need to provide the entire link of the domain
along with the sub dirs.
So i was wondering what is the way, to provide the default domain name
which will work on any domain.

for ex : if the file is lib/test.php, then a way to call it should be the
same if the domains are www.abc.com or www.xyz.com or any other domain.
something like the dirname, basename etc...

but i haven't had much luck on the same, any ideas?

Thanks,