CallbackValidator example 2.1

MyType.php

<?php

namespace Acme\DemoBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormError;
use Symfony\Component\Form\Event\DataEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class MyType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->addEventListener(FormEvents::POST_BIND, function (DataEvent $event) {
            $form = $event->getForm();
            
            if (!$form['foo']->getData()) {
                $form['foo.error']->addError(new FormError('foo.error'));
            }
        });
    }
}