code: function toXML()
      {
          $xml = new SimpleXMLElement("<?xml version=\"1.0\"?><custom_fields></custom_fields>");
          if (isset($this->custom_fields)) {
              foreach ($this->custom_fields as $index => $type) {
                  $field = $xml->addChild("field");
                  $field->addChild("name", $index);
                  $field->addChild("type", $type);
              }
          }
  
          return $xml;
      }
docs: Serialization to a simple XML element.
  
  @return \em SimpleXMLElement
  Generate a XML element from the custom fields list.