Disable backend validation for choice field in Symfony 2 Type

It's confusing but this behaviour is not really validation related as it is caused by the "ChoiceToValueTransformer" which indeed searches for entries in your pre-declared list. If your list is empty or you want to disable the transformer you can simply reset it.

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->add('yourField', 'choice', array('required'=>false));

    //more fields...

    $builder->get('yourField')->resetViewTransformers();
}

Then your custom defined validation will step in (if it exists).

Add this inside buildForm method in your form type class so that you can validate an input field value rather a choice from a select field value;

$builder->addEventListener(
    FormEvents::PRE_SUBMIT,

    function (FormEvent $event) {
        $form = $event->getForm();

        if ($form->has('field')) {
            $form->remove('field');
            $form->add(
                'field', 
                'text', 
                ['required' => false]
            )
        }
    }
);