code: public static function base($scheme = false)
      {
          $url = static::getUrlManager()->getBaseUrl();
          if ($scheme !== false) {
              $url = static::getUrlManager()->getHostInfo() . $url;
              $url = static::ensureScheme($url, $scheme);
          }
  
          return $url;
      }
docs: Returns the base URL of the current request.
  @param bool|string $scheme the URI scheme to use in the returned base URL:
  
  - `false` (default): returning the base URL without host info.
  - `true`: returning an absolute base URL whose scheme is the same as that in [[\yii\web\UrlManager::$hostInfo]].
  - string: returning an absolute base URL with the specified scheme (either `http`, `https` or empty string
  for protocol-relative URL).
  @return string