FormSubmitExtension.php

<?php

namespace CommonBundle\Twig;

use Symfony\Bridge\Twig\Form\TwigRenderer;
use Symfony\Component\Form\FormView;

class FormSubmitExtension extends \Twig_Extension
{
    /**
     * {@inheritdoc}
     */
    public function getFunctions()
    {
        return [new \Twig_SimpleFunction('form_submit', [$this, 'createSubmitButton'], ['needs_environment' => true, 'is_safe' => ['all']])];
    }

    /**
     * @param \Twig_Environment $env
     * @param FormView          $formView
     * @param string            $label
     */
    public function createSubmitButton(\Twig_Environment $env, FormView $formView, $label)
    {
        /** @var TwigRenderer $formRenderer */
        $formRenderer = $env->getExtension('form')->renderer;
        $submitView = new FormView($formView);
        $formRenderer->renderBlock($submitView, 'submit_widget', ['label' => $label]);
    }
}