tos/Form/AcceptTosType.php

<?php
namespace vierbergenlars\AuthserverTosBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\Extension\Core\Type\CheckboxType;
use Symfony\Component\Validator\Constraints\IsTrue;

class AcceptTosType extends AbstractType
{

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('accept', CheckboxType::class, [
            'label' => 'I accept the <a href="' . htmlentities($options['url'], ENT_HTML5 | ENT_QUOTES) . '">terms of service</a>',
            'attr' => [
                'align_with_widget' => true
            ],
            'constraints' => [
                new IsTrue([
                    'message' => 'You must accept the terms of service.'
                ])
            ]
        ]);
    }

    /**
     *
     * {@inheritdoc}
     * @see \Symfony\Component\Form\AbstractType::configureOptions()
     */
    public function configureOptions(\Symfony\Component\OptionsResolver\OptionsResolver $resolver)
    {
        $resolver->setDefined('url')
            ->setRequired('url')
            ->addAllowedTypes('url', 'string')
            ->setDefault('label', false);
    }
}