Symfony Custom Field or Dynamic Form?

First, you have to build a custom FormType that composes a CheckboxType and a TextType. This is the server-side form part.

But, in order to enable/disable the text field on the fly, you'll have to use Javascript.

Finally, if you want to store the result information a single nullable text field (like a nullable varchar), you need a DataTransformer to convert data from persistece layer to view.

Let's see a kind of FormType :

<?php

namespace Form\Type;

use Form\DataTransformer\NullableTextTransformer;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\CheckboxType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;

class NullableTextType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('isNotNull', CheckboxType::class)
            ->add('text', TextType::class, array('required' => false))
        ;

        $builder->addModelTransformer(new NullableTextTransformer());
    }
}

And now the transformer :

<?php

namespace Form\DataTransformer;

use Symfony\Component\Form\DataTransformerInterface;

class NullableTextTransformer implements DataTransformerInterface
{
    public function transform($value)
    {
        $nullableText = ['isNotNull' => false, 'text' => null];

        if (null !== $value) {
            $nullableText['isNotNull'] = true;
            $nullableText['text'] = (string) $value;
        }

        return $nullableText;
    }

    public function reverseTransform($array)
    {
        if (!is_array($array) || empty($array) || !isset($array['isNotNull']) || !isset($array['text'])) {
            return;
        }

        if ($array['isNotNull']) {
            return (string) $array['text'];
        } else {
            return;
        }
    }
}

Some twig for the form theming of the custom field :

{% block nullable_text_widget %}
{% spaceless %}
  <div class="input-group nullable-text-widget">
    <div class="input-group-addon">
      {{ form_widget(form.isNotNull, {attr: {class: 'is-not-null-widget'}}) }}
    </div>
    {{ form_widget(form.text, {attr: {class: 'text-widget'}}) }}
  </div>
{% endspaceless %}
{% endblock nullable_text_widget %}

And finally, a bunch of JS lines to handle the front interactions :

$(document).ready(function() {
    $.each($('body').find('.nullable-text-widget'), function () {
        syncNullableTextWidget($(this));
    });
});

$(document).on('click', '.nullable-text-widget .is-not-null-widget', function (e) {
    var $nullableTextWidget = $(e.currentTarget).parents('.nullable-text-widget');
    syncNullableTextWidget($nullableTextWidget);
});

function syncNullableTextWidget($widget)
{
    if ($widget.find('.is-not-null-widget').prop('checked')) {
        $widget.find('.text-widget').prop('disabled', false);
    } else {
        $widget.find('.text-widget').prop('disabled', true);
        $widget.find('.text-widget').val('');
    }
}