Using code dependant on domain

[Rochdalemark]

I have 2 domains using one main site and one parked alias.

The pages are php, and I need some code that will do the following

if domain is url1 then phpcode1

if domain is url2 then phpcode2

else if neither url then don't use any code.

Any help with that would be gratefully recieved.

Thanks

[Rochdalemark]

Problem Solved

<?php
if($_SERVER['HTTP_HOST'] == "www.url1.co.uk") {
    // Visiting from URL1
    echo "You are visiting from the co.uk URL";
} elseif($_SERVER['HTTP_HOST'] == "www.url2.com") {
    // Visiting from URL2
    echo "You are visiting from the .com URL";
} else {
    // Not visiting from either URL1 or URL2
    echo "You are not visiting from our .co.uk or .com URL";
}
?>