code: public function setBaseUrl($value)
      {
          $this->_baseUrl = $value === null ? null : rtrim(Yii::getAlias($value), '/');
      }
docs: Sets the base URL that is used by [[createUrl()]] to prepend to created URLs.
  This is mainly used when [[enablePrettyUrl]] is `true` and [[showScriptName]] is `false`.
  @param string $value the base URL that is used by [[createUrl()]] to prepend to created URLs.