JSON Error Handler

A structured JSON error handler for Slim Framework applications that maps exceptions to typed, consistent error payloads.

Introduction

This library provides a JSON error handler for Slim applications. It extends Slim's built-in error handling to intercept exceptions and transform them into structured JSON responses, mapping HTTP exceptions to typed error payloads with appropriate status codes. The handler supports optional error detail exposure for debug environments and integrates directly with Slim's error middleware and shutdown handling workflows.

Prerequisites

• PHP: Version 8.3 or higher is required.
• Composer: Dependency management tool for PHP.
• Slim Framework: Version 4 is required.

Installation

composer require andrewdyer/json-error-handler

Getting Started

1. Create the application

use Slim\Factory\AppFactory;

$app = AppFactory::create();

2. Add error middleware

Add the error middleware and set JsonErrorHandler as the default handler. The $displayErrorDetails flag controls whether exception messages are included in responses — this should be false in production:

use AndrewDyer\JsonErrorHandler\JsonErrorHandler;

$displayErrorDetails = true;

$errorMiddleware = $app->addErrorMiddleware(
    $displayErrorDetails,
    logErrors: true,
    logErrorDetails: true
);

$errorHandler = new JsonErrorHandler(
    $app->getCallableResolver(),
    $app->getResponseFactory(),
    logger: null
);

$errorMiddleware->setDefaultErrorHandler($errorHandler);

Note: A PSR-3 logger can be passed as the third argument to enable error logging. Monolog is a popular choice for this.

By default, payloads are encoded with JSON_PRETTY_PRINT. Custom flags can be passed as the fourth constructor argument:

$errorHandler = new JsonErrorHandler(
    $app->getCallableResolver(),
    $app->getResponseFactory(),
    logger: null,
    jsonEncodeFlags: JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES
);

Note that JSON_THROW_ON_ERROR is always masked out internally to prevent encoding failures from cascading during error handling.

3. Register routes

Register routes to handle incoming requests:

use Psr\Http\Message\ResponseInterface as Response;
use Psr\Http\Message\ServerRequestInterface as Request;
use Slim\Exception\HttpNotFoundException;

$app->get('/hello', function (Request $request, Response $response): Response {
    $response->getBody()->write(json_encode(['message' => 'Hello, world.']));
    return $response->withHeader('Content-Type', 'application/json');
});

$app->get('/error', function (Request $request, Response $response): Response {
    throw new HttpNotFoundException($request);
});

4. Run the application

Start the application to begin handling incoming HTTP requests:

$app->run();

Usage

Once the handler is registered, Slim will route exceptions through JsonErrorHandler and return structured JSON error responses.

Successful request

GET /hello
Accept: application/json

Response: 200 OK

{
  "message": "Hello, world."
}

Error request

GET /error
Accept: application/json

Response: 404 Not Found

{
  "error": {
    "type": "RESOURCE_NOT_FOUND",
    "description": "Not found."
  }
}

Advanced Usage

Shutdown handler integration

For complete fatal error coverage — including errors that occur outside of Slim's request lifecycle — JsonErrorHandler can be integrated with andrewdyer/shutdown-handler:

composer require andrewdyer/shutdown-handler

Wrap JsonErrorHandler in a CallableErrorResponder and register a ShutdownHandler before running the application:

use AndrewDyer\JsonErrorHandler\JsonErrorHandler;
use AndrewDyer\ShutdownHandler\Adapters\CallableErrorResponder;
use AndrewDyer\ShutdownHandler\Adapters\CallableResponseEmitter;
use AndrewDyer\ShutdownHandler\ShutdownHandler;
use Slim\Factory\AppFactory;
use Slim\Factory\ServerRequestCreatorFactory;
use Slim\ResponseEmitter;

$app = AppFactory::create();

$displayErrorDetails = true;

$errorMiddleware = $app->addErrorMiddleware(
    $displayErrorDetails,
    logErrors: true,
    logErrorDetails: true
);

$errorHandler = new JsonErrorHandler(
    $app->getCallableResolver(),
    $app->getResponseFactory(),
    logger: null
);

$errorMiddleware->setDefaultErrorHandler($errorHandler);

$request = ServerRequestCreatorFactory::create()->createServerRequestFromGlobals();

$responseEmitter = new ResponseEmitter();

$shutdownHandler = new ShutdownHandler(
    $request,
    new CallableErrorResponder(
        static fn ($request, $exception, bool $displayErrorDetails) => $errorHandler(
            $request,
            $exception,
            $displayErrorDetails,
            logError: true,
            logErrorDetails: true
        )
    ),
    new CallableResponseEmitter(
        static fn ($response) use ($responseEmitter) => $responseEmitter->emit($response)
    ),
    $displayErrorDetails
);

register_shutdown_function($shutdownHandler);

$response = $app->handle($request);

$responseEmitter->emit($response);

Refer to the shutdown-handler documentation for full details on implementing a response emitter.

License

Licensed under the MIT license and is free for private or commercial projects.