public function setURLCallback($className, $methodName, $parameters = array())
{
$className = (string) $className;
$methodName = (string) $methodName;
$parameters = (array) $parameters;
// store in property
$this->callback = array('class' => $className, 'method' => $methodName, 'parameters' => $parameters);
// re-load the form
$this->loadForm();
}