Slim 4 - Respect/Validation

Daniel Opitz
Daniel Opitz
08 Oct 2020

Table of contents



The Respect/Validation component is a popular and flexible validation library for PHP. It works as standalone library and without much additional dependencies.


To install Respect/Validation, run:

composer require respect/validation


I think the most common use case is to validate nested arrays. For this reason I will show how to validate a simple array with this fields:

To fetch the form data as an array use this code in your action class:

$formData = (array)$request->getParsedBody();

Then pass this array into your service for validation. Within a service class you are able to perform the validation as follows:

use Respect\Validation\Validator as v;
// ...

$validator = new v();

$validator->addRule(v::key('username', v::allOf(
    v::notEmpty()->setTemplate('The username must not be empty'),
    v::length(3, 24)->setTemplate('Invalid length')
))->setTemplate('The key "username" is required'));

$validator->addRule(v::key('email', v::allOf(
    v::notEmpty()->setTemplate('The email must not be empty'),
    v::email()->setTemplate('Invalid email address')
))->setTemplate('The key "email" is required'));

$validator->addRule(v::key('password', v::allOf(
    v::notEmpty()->setTemplate('The password must not be empty'),
    v::length(8, 60)->setTemplate('Invalid length')
))->setTemplate('The key "password" is required'));

Optional fields can be added like this. Just pass false to make the key optional:

$validator->addRule(v::key('mobile', v::allOf(
    v::notEmpty()->setTemplate('The mobile number must not be empty'),
    v::phone()->setTemplate('Invalid phone number')
), false));

To start the validation use the assert method.



To translate the strings for the error messages, just use the _ or __ function of gettext.

$validator->addRule(v::key('username', v::allOf(
    v::notEmpty()->setTemplate(__('The username must not be empty')),
    v::length(3, 24)->setTemplate(__('Invalid length'))
))->setTemplate(__('The key %s is required', 'username'));

Validation Middleware

As soon as the validation fails a NestedValidationException will be thrown.

This is great, because you can transform this specific exception into an JSON response with status code 422.

For this purpose we add a custom middleware to the stack.

Create a new file src/middleware/RespectValidationMiddleware.php and copy / paste this content:


namespace App\Middleware;

use Psr\Http\Message\ResponseFactoryInterface;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Respect\Validation\Exceptions\NestedValidationException;

final class RespectValidationMiddleware implements MiddlewareInterface
     * @var ResponseFactoryInterface
    private $responseFactory;

     * The constructor.
     * @param ResponseFactoryInterface $responseFactory The response factory
    public function __construct(ResponseFactoryInterface $responseFactory)
        $this->responseFactory = $responseFactory;

     * Invoke middleware.
     * @param ServerRequestInterface $request The request
     * @param RequestHandlerInterface $handler The handler
     * @return ResponseInterface The response
    public function process(
        ServerRequestInterface $request, 
        RequestHandlerInterface $handler
    ): ResponseInterface {
        try {
            return $handler->handle($request);
        } catch(NestedValidationException $exception) {
            $messages = [];
            /** @var ValidationException $message */
            foreach($exception->getIterator() as $message) {
                $key = $message->getParam('name');
                if($key === null) {
                $messages[$key] = $message->getMessage();
            $response = $this->responseFactory->createResponse();
            $result = [
                'error' => [
                    'message' => $exception->getMessage(),
                    'details' => $messages,
            $response->withHeader('Content-Type', 'application/json');

            return $response->withStatus(422);

In order to make it work, the RespectValidationMiddleware::class must be added after the Slim ErrorMiddleware:


use App\Middleware\RespectValidationMiddleware;

// ...
$app->addErrorMiddleware(true, true, true);
$app->add(RespectValidationMiddleware::class); // <-- here


The Respect/Validation component is very flexible and provides a lot of rules out of the box. It’s maintained, and the download number on github is impressive.

A lot of magic is going on when you use the static method of the validator class. I prefer less magic and more robust and faster code. For me a real fluent interface would be more readable and elegant, but this is just a matter of taste.

It was quite hard for me to find out how to add a custom message and translations to each validation rule. In the end it worked.

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