PHP code to get the domain name without subdomains (includes the tld, and the special types from IANA). Don't have support for unicode domain names.

get_domain_base.php

<?php
/**
 * @param string $domain Pass $_SERVER['SERVER_NAME'] here
 *
 * @return string
 *
 * replacement for https://gist.github.com/pocesar/5366899
 */

function get_domain_base($domain) {
	$original = $domain = strtolower($domain);
	if (filter_var($domain, FILTER_VALIDATE_IP)) { return $domain; }

	$tlds = array('aero','arpa','asia','biz','cat','com','coop','edu','gov','info','jobs','mil','mobi','museum','name','net','org','post','pro','tel','travel',		'xxx');

	$tlds_2 = array('govt','idf','k12','law','ltd','mod','muni','nom','nhs','parliament','plc','school','sch');

	$out = array();
	$arr = array_reverse(explode('.', $domain));
	foreach($arr as $sub) {
		if (strlen($sub)==2 && count($out) < 2) $out[] = $sub;
		elseif (in_array($sub, $tlds)) $out[] = $sub;
		elseif (in_array($sub, $tlds_2) && count($out)==1) $out[] = $sub;
		else {
			$out[] = $sub;
			break;
		}
	}
	return join('.', array_reverse($out));
}


$urls = array(
	'www.forum.example.ru' => 'example.ru',
	'www.forum.org.ru' => 'forum.org.ru',
	'www.forum.test.org.ru' => 'test.org.ru',
	'www.example.com' => 'example.com',
	'example.com' => 'example.com',
	'example.com.br' => 'example.com.br',
	'www.example.com.br' => 'example.com.br',
	'www.example.gov.br' => 'example.gov.br',
	'localhost' => 'localhost',
	'www.localhost' => 'localhost',
	'subdomain.localhost' => 'localhost',
	'www.subdomain.example.com' => 'example.com',
	'subdomain.example.com' => 'example.com',
	'subdomain.example.com.br' => 'example.com.br',
	'www.subdomain.example.com.br' => 'example.com.br',
	'www.subdomain.example.biz.br' => 'example.biz.br',
	'subdomain.example.biz.br' => 'example.biz.br',
	'subdomain.example.net' => 'example.net',
	'www.subdomain.example.net' => 'example.net',
	'www.subdomain.example.co.kr' => 'example.co.kr',
	'subdomain.example.co.kr' => 'example.co.kr',
	'example.co.kr' => 'example.co.kr',
	'example.jobs' => 'example.jobs',
	'www.example.jobs' => 'example.jobs',
	'subdomain.example.jobs' => 'example.jobs',
	'insane.subdomain.example.jobs' => 'example.jobs',
	'insane.subdomain.example.com.br' => 'example.com.br',
	'www.doubleinsane.subdomain.example.com.br' => 'example.com.br',
	'www.subdomain.example.jobs' => 'example.jobs',
	'test' => 'test',
	'www.test' => 'test',
	'subdomain.test' => 'test',
	'www.detran.sp.gov.br' => 'sp.gov.br',
	'www.mp.sp.gov.br' => 'sp.gov.br',
	'ny.library.museum' => 'library.museum',
	'www.ny.library.museum' => 'library.museum',
	'ny.ny.library.museum' => 'library.museum',
	'www.library.museum' => 'library.museum',
	'info.abril.com.br' => 'abril.com.br',
	'www.example.school.za' => 'example.school.za',
	'www.example.asd.za' => 'asd.za',
	'127.0.0.1' => '127.0.0.1',
	'www.mod.mod.uk' => 'mod.mod.uk',
	'::1' => '::1',
	'dev.example.ca' => 'example.ca',
	'dev.example.io' => 'example.io',
	'sub.example.co' => 'example.co'
);

$failed = 0;
$total = count($urls);
foreach ($urls as $from => $expected) {
	$from = get_domain_base($from);
	if ($from !== $expected) {
		$failed++;
		print("<div style='color:red;'>expected {$from} to be {$expected}, got {$from}</div>");
	}
}

if ($failed) {
	print("{$failed} tests failed out of {$total}");
} else {
	print("Success");
}