code: private function _put($attrName, $value = NULL) {
          $this->_checkModify();
  
          if($value === NULL)
              $this->removeAttribute($attrName);
          elseif($this->hasAttribute($attrName) && method_exists($attr = $this->getAttribute($attrName), "setValue")) {
              /** @var Attribute $attr */
              $attr->setValue($value);
          } else {
              $this->addAttribute(new Attribute($attrName, $value));
          }
      }
docs: Puts value into attribute
  @param $attrName
  @param null $value
  @internal