Symfony2 change form data in event

gistfile1.txt

$builder->addEventListener(
    FormEvents::PRE_SUBMIT,
    function (FormEvent $event) {
        $data = $event->getData();

        if (isset($data['url'])) {
            $url = (string) $data['url'];

            if (empty($url)) {
                $url = isset($data['title']) ? (string) $data['title'] : null;
            }

            $url = Urlizer::transliterate($url);
            $url = Urlizer::urlize($url);

            if ('/' !== $url[0]) {
                $url = "/$url";
            }

            $data['url'] = $url;
        }

        $event->setData($data);
    }
);