PHP function to get the subdomain of a URL

What I found the best and short solution is

array_shift(explode(".",$_SERVER['HTTP_HOST']));

if you are using drupal 7

this will help you:

global $base_path;
global $base_root;  
$fulldomain = parse_url($base_root);    
$splitdomain = explode(".", $fulldomain['host']);
$subdomain = $splitdomain[0];