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,