Symfony -如何将表单字段小写？

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolver;

class MyFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('myField')
            // 添加其他字段...
            ->addEventListener(FormEvents::SUBMIT, function (FormEvent $event) {
                $data = $event->getData();
                $data['myField'] = strtolower($data['myField']);
                $event->setData($data);
            });
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        // 配置表单选项...
    }
}