code: public function createAbsoluteUrl($params, $scheme = null)
      {
          $params = (array) $params;
          $url = $this->createUrl($params);
          if (strpos($url, '://') === false) {
              $hostInfo = $this->getHostInfo();
              if (strncmp($url, '//', 2) === 0) {
                  $url = substr($hostInfo, 0, strpos($hostInfo, '://')) . ':' . $url;
              } else {
                  $url = $hostInfo . $url;
              }
          }
  
          return Url::ensureScheme($url, $scheme);
      }
docs: Creates an absolute URL using the given route and query parameters.
  
  This method prepends the URL created by [[createUrl()]] with the [[hostInfo]].
  
  Note that unlike [[\yii\helpers\Url::toRoute()]], this method always treats the given route
  as an absolute route.
  
  @param string|array $params use a string to represent a route (e.g. `site/index`),
  or an array to represent a route with query parameters (e.g. `['site/index', 'param1' => 'value1']`).
  @param string|null $scheme the scheme to use for the URL (either `http`, `https` or empty string
  for protocol-relative URL).
  If not specified the scheme of the current request will be used.
  @return string the created URL
  @see createUrl()