Transformer/WrapperTransformer.php
public function transform($input, array $options = [])
    {
        $resolver = new OptionsResolver();
        $this->configureOptions($resolver);
        $options = $resolver->resolve($options);

        return [$options['wrapper_key'] => $input];
    }
public function transform($input, array $options = [])
    {
        $resolver = new OptionsResolver();
        $this->configureOptions($resolver);
        $options = $resolver->resolve($options);

        return [$options['wrapper_key'] => $input];
    }
Must return the transformed $value

@param mixed $input
@param array $options

@throws \Symfony\Component\OptionsResolver\Exception\ExceptionInterface
@throws \Exception

@return mixed $value