Eccube\Form\Type\Master\PageMaxType::buildForm PHP Method

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) {
            $options = $event->getForm()->getConfig()->getOptions();
            if (!$event->getData()) {
                $data = current(array_keys($options['choice_list']->getValues()));
                $event->setData($data);
            }
        });
        $builder->addEventListener(FormEvents::PRE_SUBMIT, function (FormEvent $event) {
            $options = $event->getForm()->getConfig()->getOptions();
            $values = $options['choice_list']->getValues();
            if (!in_array($event->getData(), $values)) {
                $data = current($values);
                $event->setData($data);
            }
        });
    }