Reading data in address bar

Hi
Can anyone help me with a problem?
If it is possible to do like this?
I need help with a solution where I can send someone to
<test>.domain.com.

(Or, if not possible - http://domain.com/<test>)
where <test> is a value defined in a mysql database.
For example, if I write: http://test.mydomain.com
then it check if "test" exist in the database.
If exist, redirect to mydomain.com/defined_page and session=ok.
If not, redirect to mydomain.com/not_defined_page and session=not_ok

The checking in database, or setting of session are no problem.
Just the reading.

----in use, but I need an easier/shorter address-----
address = http://mydomain.com/page.php&value=<test>

require "functions.php";
if ( isset($_GET['value'])){
$get_test_ok = checkHtest($_GET['value']);
if($get_test_ok){
$exist = "1";
$_SESSION['sess_exist'] = $exist;
$_SESSION['sess_test'] = $_GET['value'];
$testId = gettestId($_SESSION['sess_test']);}
----This was not so hard to create, but..-----

Is this possible to use http://<test>.mydomain.com?
or http://mydomain.com/<test>?
What must be done on my webserver?

Hope someone can help me

Best regards
Sven Josefsson