example.php

<?php
//...
$builder->add('recaptcha', 'recaptcha', array(
    'private_key' => 'private_key_here_required',
    'public_key' => 'public_key_here_required',
))
//...

gaming.xml

<service id="form.type.recaptcha" class="Comways\GamingBundle\Form\Type\RecaptchaType" scope="request">
    <tag name="form.type" alias="recaptcha" />
    <argument type="service" id="request" />
</service>

recaptcha.html.twig

{% block recaptcha_widget %}
{% spaceless %}
    <script type="text/javascript">
        var RecaptchaOptions = {{ widget_options|default({})|json_encode|raw }};
    </script>

    <script type="text/javascript" src="http://www.google.com/recaptcha/api/challenge?k={{ public_key }}"></script>
    <noscript>
        <iframe src="http://www.google.com/recaptcha/api/noscript?k={{ public_key }}" height="300" width="500" frameborder="0"></iframe>
        {{ form_widget(form.recaptcha_challenge_field) }}
        {{ form_widget(form.recaptcha_response_field) }}
    </noscript>
{% endspaceless %}
{% endblock %}

RecaptchaTransformer.php

<?php

namespace Comways\GamingBundle\Form\DataTransformer;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Form\DataTransformer\TransformationFailedException;

/**
 * Transforms the request into the right fields for validating the
 * recaptcha field.
 *
 * @author Henrik Bjornskov <henrik@bjrnskov.dk>
 */
class RecaptchaTransformer implements \Symfony\Component\Form\DataTransformerInterface
{
    /**
     * @var Request
     */
    protected $request;

    /**
     * @var string
     */
    protected $privateKey;

    /**
     * @param Request $request
     * @param string  $privateKey
     */
    public function __construct(Request $request, $privateKey)
    {
        $this->request    = $request;
        $this->privateKey = $privateKey;
    }

    /**
     * @param  mixed $value
     * @return mixed
     */
    public function transform($value)
    {
        return $value;
    }

    /**
     * Transforms the value from the request and validates the fields according to the api
     *
     * @param  array $array
     * @return array
     */
    public function reverseTransform($array)
    {
        $request = $this->request->request;
        $params  = array(
            'remoteip'   => $this->request->server->get('REMOVE_ADDR', '127.0.0.1'),
            'privatekey' => $this->privateKey,
            'challenge'  => $request->get('recaptcha_challenge_field', $array['recaptcha_challenge_field']), 
            'response'   => $request->get('recaptcha_response_field', $array['recaptcha_response_field']),
        );

        $context = stream_context_create(array(
            'http' => array(
                'method'  => 'POST',
                'content' => http_build_query($params),
                'header'  => array(
                    'Content-Type: application/x-www-form-urlencoded',
                ),
            ),
        ));

        $result = file_get_contents('http://www.google.com/recaptcha/api/verify', false, $context);

        if (false !== strpos($result, 'true', 0)) {
            return array(
                'recaptcha_response_field'  => 'manual_challenge',
                'recaptcha_challenge_field' => '',
            );
        }

        throw new TransformationFailedException('The entered Recaptcha code is invalid');
    }
}

RecaptchaType.php

<?php

namespace Comways\GamingBundle\Form\Type;

use Symfony\Component\Form\FormBuilder;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\HttpFoundation\Request;

use Comways\GamingBundle\Form\DataTransformer\RecaptchaTransformer;

/**
 * A ReCaptcha type for use with Google ReCatpcha services. It embeds two fields that are used
 * for manual validation and show of the widget.
 *
 * The DataTransformer takes the entered request information and validates them agains the 
 * Google Recaptcha API.
 *
 * example:
 *     $builder->add('recaptcha', 'recaptcha', array(
 *         'private_key' => 'private_key_here_required',
 *         'public_key' => 'public_key_here_required',
 *     ))
 *
 *
 * @author Henrik Bjornskov <henrik@bjrnskov.dk>
 */
class RecaptchaType extends \Symfony\Component\Form\AbstractType
{
    /**
     * @var Request
     */
    protected $request;

    /**
     * @param Request $request
     */
    public function __construct(Request $request)
    {
        $this->request = $request;
    }

    /**
     * Configures the Type
     *
     * @param FormBuilder $builder
     * @param array       $options
     */
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('recaptcha_challenge_field', 'text')
            ->add('recaptcha_response_field', 'hidden', array(
                'data' => 'manual_challenge',
            ))
        ;

        $builder->prependClientTransformer(new RecaptchaTransformer($this->request, $options['private_key']));

        $builder
            ->setAttribute('public_key', $options['public_key'])
            ->setAttribute('widget_options', $options['widget_options'])
        ;
    }

    /**
     * Options for this type
     *
     * @param  array $options
     * @return array
     */
    public function getDefaultOptions(array $options)
    {
        return array(
            'csrf_protection' => false,
            'required'        => false,
            'error_bubbling'  => false,
            'property_path'   => false,
            'private_key'     => null,
            'public_key'      => null,
            'widget_options'  => array(),
        );
    }

    /**
     * Sets attributes for use with the renderer
     *
     * @param FormView $view
     * @param FormInterface $form
     */
    public function buildView(FormView $view, FormInterface $form)
    {
        $view->set('public_key', $form->getAttribute('public_key'));
        $view->set('widget_options', $form->getAttribute('widget_options'));
    }

    /**
     * Because this have property_path = null and it shouldnt be written this parent
     * is a field.
     *
     * @return string
     */
    public function getParent(array $options)
    {
        return 'form';
    }

    /**
     * Used to identify the rendering block
     *
     * @return string
     */
    public function getName()
    {
        return 'recaptcha';
    }
}