get url (including subdomain)

you have a string with the whole URL on the variable:

$_SERVER['PHP_SELF']

then you can split string by using php functions to get different parts of string. Functions like strpos to get the position of a character inside an string, or functions like substr to get a part of the string.

Here you have the list of all string functions:
http://www.php.net/manual/en/ref.strings.php

Hope it helps.

I generally agree with Medo3777, however the URL might be something like www.atf77.whitehouse.gov/csm/index.php?q=foo.bar so you might want to learn about the PHP functions that parse strings and parse urls.  Google those terms and you will find a wealth of generalized solutions for parsing this sort of information.

i will have to open a question about a complex url that Ray suggested