You can declare it like so (In functions.php):

global $domain;
$domain = 'http://www.mydomain.com/';

You can echo the domain of the WordPress site like this, though..

<?php bloginfo('site_url');?>

Or

$url = site_url();
echo $url;

Or as a function, in functions.php

function displayMyDomain() {
  global $domain; // you probably don't actually need to set it global as it is a function
  $domain = 'http://www.domain.com/';
  echo $domain;
}

Then anywhere, use <?php displayMyDomain(); ?>