code: public static function handleUnknownHost(URL $webroot, URL $request, array $sites, Dictionary $cfg)
      {
          // Determine behaviour on unknown host
          $on_unknown = strtoupper($cfg->dget('unknown_host_policy', "IGNORE"));
          $best_matching = self::findBestMatching($webroot, $sites);
  
          if ($on_unknown === "ERROR" || ($best_matching === null && $on_unknown === "REDIRECT"))
              return null;
  
          if ($on_unknown === "REDIRECT")
          {
              $redir = $best_matching->URL($request->path);
              return $redir;
          }
  
          // Generate a proper VirtualHost on the fly
          $url = new URL($webroot);
          $url->fragment = null;
          $url->query = null;
          $lang = $cfg->dget('default_language', 'en');
          $vhost = new VirtualHost($url, $lang);
  
          // Add the new virtualhost to a site.
          if ($best_matching === null)
          {
              // If no site has been defined, create a new one
              $site = new Site();
              $site->addVirtualHost($vhost);
          }
          else
              $best_matching->getSite()->addVirtualHost($vhost);
  
          return $vhost;
      }
docs: Determine what to do when a request was made to an unknown host.
  
  The default configuration is IGNORE, which means that a new vhost will be
  generated on the fly and attached to the site of the closest matching VirtualHost.
  If no site is configured either, a new Site named 'defaul't is created and the new
  VirtualHost is attached to that site. This makes configuration non-required for
  simple sites with one site and one hostname.
  
  @param URL $webroot The webroot where a request was made
  @param URL $url The URL that was requested
  @param array $sites The configured sites
  @param Dictionary $cfg The configuration to get the policy from
  @return mixed One of:
  * null: if the policy is to error out on unknown hosts
  * URI: if the policy is to redirect to the closest matching host
  * VirtualHost: if the policy is to ignore / accept unknown hosts