gistfile1.php

function getBaseDomain($domain = '') {
	$result = false;
	
	if($domain) {
		$url = parse_url($domain);
		if(isset($url['host']) && $url['host']) {
			$parts = explode('.', $url['host']);
			
			$middlePart = array_slice($parts, -2, 1)[0];
			$slice = ((strlen($middlePart) <= 3) && (count($parts) > 2)) ? 3 : 2;
			
			$result = implode('.', array_slice($parts, (0 - $slice), $slice));
		}
	}
	
	return $result;
}

nice job

make support for php 5.3+

$middlePart = array_slice($parts, -2, 1);
$mid = $middlePart[0];
$slice = ((strlen($mid) <= 3) && (count($parts) > 2)) ? 3 : 2;