$input = 'www.google.co.uk/';
// in case scheme relative URI is passed, e.g., //www.google.com/
$input = trim($input, '/');
// If scheme not included, prepend it
if (!preg_match('#^http(s)?://#', $input)) {
$input = 'http://' . $input;
}
$urlParts = parse_url($input);
// remove www
$domain = preg_replace('/^www\./', '', $urlParts['host']);
echo $domain;
// output: google.co.uk