Symfony DataTransformer for decimals (number) with comma and decimal points

DecimalType.php

<?php

namespace AppBundle\Form;

use AppBundle\Form\DataTransformer\NumberToStringTransformer;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;

/**
 * Reusable DecimalType Field with usage of NumberToStringTransformer
 *
 * @see http://symfony.com/doc/3.4/form/data_transformers.html#creating-a-reusable-issue-selector-field
 */
class DecimalType extends AbstractType
{
    /**
     * @var NumberToStringTransformer 
     */
    private $numberToStringTransformer;

    public function __construct(NumberToStringTransformer $numberToStringTransformer)
    {
        $this->numberToStringTransformer = $numberToStringTransformer;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->addModelTransformer($this->numberToStringTransformer);
    }

    public function getParent()
    {
        return TextType::class;
    }
}

NumberToStringTransformer.php

<?php

namespace AppBundle\Form\DataTransformer;

use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\Form\Exception\UnexpectedTypeException;
use Symfony\Component\Form\Exception\TransformationFailedException;

/**
 * A simple number transformer that allows either comma or dot as decimal separator.
 *
 * Caution: this transformer does not understand thousands separators.
 *
 * @see http://stackoverflow.com/questions/12026050/how-to-create-a-number-field-that-accepts-numbers-with-comma-or-period-in-symfon
 * @see https://gist.github.com/chrif/3411067
 */
class NumberToStringTransformer implements DataTransformerInterface
{
    /**
     * @param mixed $value
     * @return string
     */
    public function transform($value)
    {
        if ($value === null) {
            return '';
        }

        if (!is_numeric($value)) {
            throw new UnexpectedTypeException($value, 'numeric');
        }

        $value = str_replace('.', ',', $value);

        return (string)$value;
    }

    /**
     * @param mixed $value
     * @return float|mixed|null
     */
    public function reverseTransform($value)
    {
        if (!$value || $value === '') {
            return null;
        }

        if (!is_string($value)) {
            throw new UnexpectedTypeException($value, 'string');
        }

        $value = str_replace(',', '.', $value);

        if (is_numeric($value)) {
            $value = (float)$value;
        } else {
            throw new TransformationFailedException('not a number');
        }

        return $value;
    }
}