Get a domain without its subdomain (taking ccSLD into consideration)

get-domain-with-ccsld.php

<?php
// composer require jeremykendall/php-domain-parser
require_once __DIR__ . '/vendor/autoload.php';
$manager = new Pdp\Manager(new Pdp\Cache(), new Pdp\CurlHttpClient());
$domain = $manager->getRules()->resolve('this.is.a.very.long.domain.co.uk');
var_dump($domain->getRegistrableDomain());