code: private static function getBaseUrl(): ?string
      {
          $serverName = filter_input(
              \INPUT_SERVER,
              'SERVER_NAME',
              \FILTER_SANITIZE_STRING
          );
          if (!empty($serverName)) {
              $https = filter_input(\INPUT_SERVER, 'HTTPS', \FILTER_SANITIZE_STRING);
              $protocol = !empty($https) && strtolower($https) === 'on' ? 'https'
                  : 'http';
              return $protocol.'://'.$serverName;
          }
          return null;
      }
docs: It determines the basic site URL.
  @return string|null