$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

可以正确使用所有示例输入.

Works correctly with all your example inputs.