Symfony_api_normalizer.php

<?php

class APINormalizer implements NormalizerInterface
{
    public function normalize($exception, string $format = null, array $context = [])
    {
        if($exception instanceof NotFoundHttpException){
            return [
                'exception'=> [
                    'message' => $exception->getMessage(),
                    'code' => $exception->getStatusCode(),
                ],
            ];
        }
        return [
            'content' => 'This is my custom problem normalizer.',
            'exception'=> [
                'message' => $exception->getMessage(),
                'code' => $exception->getStatusCode(),
            ],
        ];
    }

    public function supportsNormalization($data, string $format = null)
    {
        return $data instanceof FlattenException;
    }
}