code: protected function truncateHost($host)
      {
          $tlds = SimpleUrl::getAllTopLevelDomains();
          if (preg_match('/[a-z\-]+\.(' . $tlds . ')$/i', $host, $matches)) {
              return $matches[0];
          } elseif (preg_match('/[a-z\-]+\.[a-z\-]+\.[a-z\-]+$/i', $host, $matches)) {
              return $matches[0];
          }
  
          return false;
      }
docs: Extracts just the domain part that determines a cookie's host validity.
  
  @param string $host Host name to truncate.
  
  @return string Domain or false on a bad host.