Exmple JsonErrorHandler for Silex (or any other app implementing HttpKernel)

app.php

<?php

// Register the error handler
$errorHandler = new JsonErrorHandler($app);
$app->error(array($errorHandler, 'handle'));

JsonErrorHandler.php

<?php

namespace Aptoma;

use Silex\Application;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Exception\HttpException;

/**
 * JsonErrorHandler is able to capture exceptions and do smart stuff with them.
 *
 * The current implementation only formats the response as JSON if the request
 * accepts JSON as content type.
 *
 * @author Gunnar Lium <gunnar@aptoma.com>
 */
class JsonErrorHandler
{
    /**
     * @var Application
     */
    private $app;

    /**
     * @var Request
     */
    private $request;

    public function __construct(Application $app)
    {
        $this->app = $app;
    }

    public function setRequest(Request $request)
    {
        $this->request = $request;

        return $this;
    }

    public function handle(HttpException $e, $code)
    {
        if (!$this->request) {
            try {
                $this->request = $this->app['request'];
            } catch (\RuntimeException $e) {
                return null;
            }
        }

        if (!in_array('application/json', $this->request->getAcceptableContentTypes())) {
            return null;
        }

        $message = array(
            'status' => $e->getStatusCode(),
            'code' => $code,
            'message' => $e->getMessage()
        );

        return $this->app->json(
            $message,
            $e->getStatusCode(),
            $e->getHeaders()
        );
    }
}