[Halfdeck]

function base_domain($url) { // parse out subdomains
		
	$url = parse_url($url);
	$host = $url['host'];
		
	$pattern = "/([\-a-z0-9]*\.)?([\-a-z0-9]*)\.([a-z0-9]*)/i";
	preg_match($pattern, $host, $matches);
	$host = $matches[2] ."." .$matches[3];

	return $host;
}

Gotta be modified for stuff like .co.uk I guess.