gistfile1.php

<?php

use Doctrine\Common\Annotations\AnnotationRegistry;
use Symfony\Component\Form\Extension\Validator\ValidatorExtension;
use Symfony\Component\Form\Forms;
use Symfony\Component\Validator\Constraints\Callback;
use Symfony\Component\Validator\Context\ExecutionContextInterface;
use Symfony\Component\Validator\Validation;

$loader = require_once __DIR__.'/vendor/autoload.php';

AnnotationRegistry::registerLoader(array($loader, 'loadClass'));

class CallbackTest
{
    /**
     * @param                           $constraint
     * @param ExecutionContextInterface $context
     *
     * @Callback(groups="MyGroup")
     */
    public static function validate($constraint, $context)
    {
        var_dump('foo');
        die;
    }
}

$validator = Validation::createValidatorBuilder()
    ->setApiVersion(Validation::API_VERSION_2_4)
    ->enableAnnotationMapping()
    ->getValidator();

$formFactory = Forms::createFormFactoryBuilder()
    ->addExtension(new ValidatorExtension($validator))
    ->getFormFactory();

$options = array(
    'data_class' => '\CallbackTest',
    'validation_groups' => 'MyGroup',
);

$object = new CallbackTest();

$form = $formFactory->createBuilder('form', $object, $options)
    ->add('foo', 'text', array('mapped' => false))
    ->getForm();

$form->submit(null);