code: public function getHostName()
      {
          if ($this->_hostName === null) {
              $this->_hostName = parse_url($this->getHostInfo(), PHP_URL_HOST);
          }
  
          return $this->_hostName;
      }
docs: Returns the host part of the current request URL.
  Value is calculated from current [[getHostInfo()|hostInfo]] property.
  
  > Warning: The content of this value may not be reliable, dependent on the server
  > configuration. Please refer to [[getHostInfo()]] for more information.
  
  @return string|null hostname part of the request URL (e.g. `www.yiiframework.com`)
  @see getHostInfo()
  @since 2.0.10