code: public function getBaseUrl()
      {
          if ($this->_baseUrl === null) {
              $this->_baseUrl = rtrim(dirname($this->getScriptUrl()), '\\/');
          }
  
          return $this->_baseUrl;
      }
docs: Returns the relative URL for the application.
  This is similar to [[scriptUrl]] except that it does not include the script file name,
  and the ending slashes are removed.
  @return string the relative URL for the application
  @see setScriptUrl()