Php get domain without subdomain

It is not possible to get the domain name without using a TLD list to compare with as their exist many cases with completely the same structure and length:

 nas.db.de (Subdomain)
 bbc.co.uk (Top-Level-Domain)

 www.uk.com (Subdomain)
 big.uk.com (Second-Level-Domain)

Mozilla's public suffix list should be the best option as it is used by all major browsers:
https://publicsuffix.org/list/public_suffix_list.dat

Feel free to use my function:

function tld_list($cache_dir=null) {
    // we use "/tmp" if $cache_dir is not set
    $cache_dir = isset($cache_dir) ? $cache_dir : sys_get_temp_dir();
    $lock_dir = $cache_dir . '/public_suffix_list_lock/';
    $list_dir = $cache_dir . '/public_suffix_list/';
    // refresh list all 30 days
    if (file_exists($list_dir) && @filemtime($list_dir) + 2592000 > time()) {
        return $list_dir;
    }
    // use exclusive lock to avoid race conditions
    if (!file_exists($lock_dir) && @mkdir($lock_dir)) {
        // read from source
        $list = @fopen('https://publicsuffix.org/list/public_suffix_list.dat', 'r');
        if ($list) {
            // the list is older than 30 days so delete everything first
            if (file_exists($list_dir)) {
                foreach (glob($list_dir . '*') as $filename) {
                    unlink($filename);
                }
                rmdir($list_dir);
            }
            // now set list directory with new timestamp
            mkdir($list_dir);
            // read line-by-line to avoid high memory usage
            while ($line = fgets($list)) {
                // skip comments and empty lines
                if ($line[0] == '/' || !$line) {
                    continue;
                }
                // remove wildcard
                if ($line[0] . $line[1] == '*.') {
                    $line = substr($line, 2);
                }
                // remove exclamation mark
                if ($line[0] == '!') {
                    $line = substr($line, 1);
                }
                // reverse TLD and remove linebreak
                $line = implode('.', array_reverse(explode('.', (trim($line)))));
                // we split the TLD list to reduce memory usage
                touch($list_dir . $line);
            }
            fclose($list);
        }
        @rmdir($lock_dir);
    }
    // repair locks (should never happen)
    if (file_exists($lock_dir) && mt_rand(0, 100) == 0 && @filemtime($lock_dir) + 86400 < time()) {
        @rmdir($lock_dir);
    }
    return $list_dir;
}
function get_domain($url=null) {
    // obtain location of public suffix list
    $tld_dir = tld_list();
    // no url = our own host
    $url = isset($url) ? $url : $_SERVER['SERVER_NAME'];
    // add missing scheme      ftp://            http:// ftps://   https://
    $url = !isset($url[5]) || ($url[3] != ':' && $url[4] != ':' && $url[5] != ':') ? 'http://' . $url : $url;
    // remove "/path/file.html", "/:80", etc.
    $url = parse_url($url, PHP_URL_HOST);
    // replace absolute domain name by relative (http://www.dns-sd.org/TrailingDotsInDomainNames.html)
    $url = trim($url, '.');
    // check if TLD exists
    $url = explode('.', $url);
    $parts = array_reverse($url);
    foreach ($parts as $key => $part) {
        $tld = implode('.', $parts);
        if (file_exists($tld_dir . $tld)) {
            return !$key ? '' : implode('.', array_slice($url, $key - 1));
        }
        // remove last part
        array_pop($parts);
    }
    return '';
}

What it makes special:

• it accepts every input like URLs, hostnames or domains with- or without scheme
• the list is downloaded row-by-row to avoid high memory usage
• it creates a new file per TLD in a cache folder so get_domain() only needs to check through file_exists() if it exists so it does not need to include a huge database on every request like TLDExtract does it.
• the list will be automatically updated every 30 days

Test:

$urls = array(
    'http://www.example.com',// example.com
    'http://subdomain.example.com',// example.com
    'http://www.example.uk.com',// example.uk.com
    'http://www.example.co.uk',// example.co.uk
    'http://www.example.com.ac',// example.com.ac
    'http://example.com.ac',// example.com.ac
    'http://www.example.accident-prevention.aero',// example.accident-prevention.aero
    'http://www.example.sub.ar',// sub.ar
    'http://www.congresodelalengua3.ar',// congresodelalengua3.ar
    'http://congresodelalengua3.ar',// congresodelalengua3.ar
    'http://www.example.pvt.k12.ma.us',// example.pvt.k12.ma.us
    'http://www.example.lib.wy.us',// example.lib.wy.us
    'com',// empty
    '.com',// empty
    'http://big.uk.com',// big.uk.com
    'uk.com',// empty
    'www.uk.com',// www.uk.com
    '.uk.com',// empty
    'stackoverflow.com',// stackoverflow.com
    '.foobarfoo',// empty
    '',// empty
    false,// empty
    ' ',// empty
    1,// empty
    'a',// empty    
);

Recent version with explanations (German):
http://www.programmierer-forum.de/domainnamen-ermitteln-t244185.htm