code: public function display($level = 0)
      {
          $value = $this->getContent();
  
          if (null === $value) {
              $value = 'null';
          } elseif (is_object($value)) {
              $value = get_class($value);
          } elseif (is_array($value)) {
              $value = 'Array';
          }
  
          $ret = str_repeat(' ', $level * 4) . $value . "\n";
  
          $children = $this->getChildren();
  
          foreach ($children as $child) {
              $ret .= $child->display($level + 1);
          }
  
          return $ret;
      }
docs: Displays all elements from a tree in hierarchic order
  
  @param int $level Current level of indentation