controller.php

<?php
...
        $form = $this->createForm(new ContactType, new \test);

        $form->bind((array('_token'=>'2f988dd5bee5917e4c7251ccce845885906eeea5',
                'amount' => '123')));
        var_dump($form->isValid()); // true
        return;

test.php

<?php

class test {
    public $amount;
}

type.php

<?php


namespace App\AppBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilder;

class ContactType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder->add('amount', 'choice', array('choices' => array(
                'test'    =>'test'
            )));
    }

    /**
     * {@inheritdoc}
     */
    public function getName()
    {
        return 'contact';
    }
}