Symfony/Silex data transformer, in reference to https://twitter.com/dragonmantank/status/599399796181487616

ArrayToStringTransformer.php

<?php

namespace App\Form\DataTransformer;

use Symfony\Component\Form\DataTransformerInterface;

class ArrayToStringTransformer implements DataTransformerInterface
{
    public function transform($value)
    {
        if (!is_array($value)) {
            return '';
        }
        return join(', ', $value);
    }
    public function reverseTransform($value)
    {
        return array_map('trim', explode(',', $value));
    }
}

ItemType.php

<?php

namespace App\Form\Type;

use Symfony\Component\Form\AbstractType;
use App\Form\DataTransformer\ArrayToStringTransformer;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Validator\Constraints as Assert;

class VideoType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add(
                'title',
                'text',
                array(
                    'attr' => array('class' => 'spans6'),
                    'constraints' => array(new Assert\NotBlank())
                )
            )
            ->add($builder
                ->create('tags', 'text', array('required' => false, 'attr' => array('class' => 'spans6')))
                ->addModelTransformer(new ArrayToStringTransformer())
            );
    }

    public function getName()
    {
        return 'item';
    }
}