MoneyType.php

class MoneyType extends AbstractType implements DataMapperInterface
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('amount', 'integer')
            ->add('currency', 'string')
            ->setDataMapper($this)
        ;
    }
    
    public function mapDataToForms($data, $forms)
    {
        foreach ($forms as $form) {
            switch ($form->getName()) {
                case 'amount':
                    $form->setData($data->getAmount());
                    break;
                case 'currency':
                    $form->setData($data->getCurrency());
                    break;
            }
        }
    }
    
    public function mapFormsToData($forms, &$data)
    {
        $forms = iterator_to_array($forms);
        $data = new Money($forms['amount']->getData(), $forms['currency']->getData());
    }
}

I wrote a more detailed blog post about this now.