Symfony Interview Questions: Top 25 in 2026

// Entry point for all HTTP requests
use App\Kernel;

require_once dirname(__DIR__).'/vendor/autoload_runtime.php';

// Symfony Runtime handles the bootstrap
return function (array $context) {
    return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
};

// The Kernel orchestrates request processing
namespace App;

use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    // The Kernel loads bundles and configures the container
    // Key lifecycle events:
    // 1. kernel.request - Before routing
    // 2. kernel.controller - After controller resolution
    // 3. kernel.view - If controller doesn't return a Response
    // 4. kernel.response - Before sending the response
    // 5. kernel.terminate - After sending (async tasks)
}

// Service with automatically injected dependencies
namespace App\Service;

use App\Repository\OrderRepository;
use Psr\Log\LoggerInterface;
use Symfony\Contracts\HttpClient\HttpClientInterface;

class PaymentService
{
    public function __construct(
        private readonly HttpClientInterface $stripeClient, // Configured HTTP client
        private readonly OrderRepository $orderRepository,   // Doctrine repository
        private readonly LoggerInterface $logger,            // PSR-3 logger
        private readonly string $stripeApiKey,               // Injected parameter
    ) {}

    public function processPayment(int $orderId, float $amount): bool
    {
        $order = $this->orderRepository->find($orderId);

        try {
            $response = $this->stripeClient->request('POST', '/charges', [
                'body' => [
                    'amount' => $amount * 100,
                    'currency' => 'eur',
                    'source' => $order->getPaymentToken(),
                ],
            ]);

            $order->markAsPaid($response->toArray()['id']);
            $this->orderRepository->save($order, true);

            return true;
        } catch (\Exception $e) {
            $this->logger->error('Payment failed', [
                'order' => $orderId,
                'error' => $e->getMessage(),
            ]);
            return false;
        }
    }
}

# config/services.yaml
# Service configuration
services:
    _defaults:
        autowire: true      # Automatic injection by type-hint
        autoconfigure: true # Automatic tag configuration

    App\:
        resource: '../src/'
        exclude:
            - '../src/DependencyInjection/'
            - '../src/Entity/'
            - '../src/Kernel.php'

    # Explicit configuration with parameters
    App\Service\PaymentService:
        arguments:
            $stripeClient: '@stripe.client'
            $stripeApiKey: '%env(STRIPE_API_KEY)%'

// Custom Bundle structure
namespace App\MyBundle;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
use Symfony\Component\HttpKernel\Bundle\AbstractBundle;

class MyBundle extends AbstractBundle
{
    // Load bundle configuration
    public function loadExtension(
        array $config,
        ContainerConfigurator $container,
        ContainerBuilder $builder
    ): void {
        // Load bundle services
        $container->import('../config/services.yaml');

        // Conditional configuration
        if ($config['feature_enabled']) {
            $container->services()
                ->set('my_bundle.feature_service', FeatureService::class)
                ->autowire();
        }
    }

    // Default bundle configuration
    public function configure(DefinitionConfigurator $definition): void
    {
        $definition->rootNode()
            ->children()
                ->booleanNode('feature_enabled')->defaultTrue()->end()
                ->scalarNode('api_key')->isRequired()->end()
            ->end();
    }
}

// Using a Component without Symfony
// Components are standalone PHP libraries
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

// Usable in any PHP project
$request = Request::createFromGlobals();
$response = new Response('Hello World', 200);
$response->send();

namespace App\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;
use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
use Symfony\Component\HttpKernel\KernelEvents;

class ApiExceptionSubscriber implements EventSubscriberInterface
{
    // Declare listened events and their priority
    public static function getSubscribedEvents(): array
    {
        return [
            // High priority (executed before others)
            KernelEvents::EXCEPTION => ['onKernelException', 100],
            KernelEvents::RESPONSE => ['onKernelResponse', 0],
        ];
    }

    public function onKernelException(ExceptionEvent $event): void
    {
        $exception = $event->getThrowable();
        $request = $event->getRequest();

        // Only handle API requests
        if (!str_starts_with($request->getPathInfo(), '/api')) {
            return;
        }

        $statusCode = $exception instanceof HttpExceptionInterface
            ? $exception->getStatusCode()
            : 500;

        $response = new JsonResponse([
            'error' => true,
            'message' => $exception->getMessage(),
            'code' => $statusCode,
        ], $statusCode);

        // Replace response with our JSON
        $event->setResponse($response);
    }

    public function onKernelResponse(\Symfony\Component\HttpKernel\Event\ResponseEvent $event): void
    {
        // Add custom headers
        $event->getResponse()->headers->set('X-Api-Version', '1.0');
    }
}

namespace App\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity(repositoryClass: UserRepository::class)]
class User
{
    #[ORM\Id]
    #[ORM\GeneratedValue]
    #[ORM\Column]
    private ?int $id = null;

    // OneToOne relation: a user has one profile
    #[ORM\OneToOne(mappedBy: 'user', cascade: ['persist', 'remove'])]
    private ?Profile $profile = null;

    // OneToMany relation: a user has multiple articles
    #[ORM\OneToMany(mappedBy: 'author', targetEntity: Article::class, orphanRemoval: true)]
    private Collection $articles;

    // ManyToMany relation: multiple users have multiple roles
    #[ORM\ManyToMany(targetEntity: Role::class, inversedBy: 'users')]
    #[ORM\JoinTable(name: 'user_roles')]
    private Collection $roles;

    public function __construct()
    {
        // Mandatory collection initialization
        $this->articles = new ArrayCollection();
        $this->roles = new ArrayCollection();
    }

    public function addArticle(Article $article): static
    {
        if (!$this->articles->contains($article)) {
            $this->articles->add($article);
            $article->setAuthor($this); // Bidirectional synchronization
        }
        return $this;
    }

    public function removeArticle(Article $article): static
    {
        if ($this->articles->removeElement($article)) {
            if ($article->getAuthor() === $this) {
                $article->setAuthor(null);
            }
        }
        return $this;
    }
}

#[ORM\Entity(repositoryClass: ArticleRepository::class)]
class Article
{
    // ManyToOne relation: multiple articles belong to one author
    #[ORM\ManyToOne(inversedBy: 'articles')]
    #[ORM\JoinColumn(nullable: false)]
    private ?User $author = null;

    // ManyToMany with additional attributes via pivot entity
    #[ORM\OneToMany(mappedBy: 'article', targetEntity: ArticleTag::class)]
    private Collection $articleTags;
}

namespace App\Repository;

use App\Entity\Article;
use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
use Doctrine\Persistence\ManagerRegistry;

class ArticleRepository extends ServiceEntityRepository
{
    public function __construct(ManagerRegistry $registry)
    {
        parent::__construct($registry, Article::class);
    }

    // BAD: N+1 queries if accessing authors
    public function findAllBad(): array
    {
        return $this->findAll();
        // + 1 query per article to load the author
    }

    // GOOD: JOIN with eager fetch
    public function findAllWithAuthor(): array
    {
        return $this->createQueryBuilder('a')
            ->addSelect('u')              // SELECT author as well
            ->leftJoin('a.author', 'u')   // JOIN on the relation
            ->orderBy('a.publishedAt', 'DESC')
            ->getQuery()
            ->getResult();
    }

    // GOOD: Multiple JOINs for multiple relations
    public function findAllWithDetails(): array
    {
        return $this->createQueryBuilder('a')
            ->addSelect('u', 'c', 't')    // SELECT all relations
            ->leftJoin('a.author', 'u')
            ->leftJoin('a.comments', 'c')
            ->leftJoin('a.tags', 't')
            ->where('a.status = :status')
            ->setParameter('status', 'published')
            ->orderBy('a.publishedAt', 'DESC')
            ->getQuery()
            ->getResult();
    }

    // GOOD: Batch loading for large lists
    public function findAllOptimized(): array
    {
        $query = $this->createQueryBuilder('a')
            ->getQuery();

        // Load relations in batches of 100
        $query->setFetchMode(Article::class, 'author', ClassMetadata::FETCH_BATCH);

        return $query->getResult();
    }
}

// API Platform extension to automatically filter by user
namespace App\Doctrine\Extension;

use ApiPlatform\Doctrine\Orm\Extension\QueryCollectionExtensionInterface;
use ApiPlatform\Doctrine\Orm\Extension\QueryItemExtensionInterface;
use ApiPlatform\Doctrine\Orm\Util\QueryNameGeneratorInterface;
use ApiPlatform\Metadata\Operation;
use App\Entity\Article;
use Doctrine\ORM\QueryBuilder;
use Symfony\Bundle\SecurityBundle\Security;

final class CurrentUserExtension implements QueryCollectionExtensionInterface, QueryItemExtensionInterface
{
    public function __construct(
        private readonly Security $security,
    ) {}

    public function applyToCollection(
        QueryBuilder $queryBuilder,
        QueryNameGeneratorInterface $queryNameGenerator,
        string $resourceClass,
        ?Operation $operation = null,
        array $context = []
    ): void {
        $this->addWhere($queryBuilder, $resourceClass);
    }

    public function applyToItem(
        QueryBuilder $queryBuilder,
        QueryNameGeneratorInterface $queryNameGenerator,
        string $resourceClass,
        array $identifiers,
        ?Operation $operation = null,
        array $context = []
    ): void {
        $this->addWhere($queryBuilder, $resourceClass);
    }

    private function addWhere(QueryBuilder $queryBuilder, string $resourceClass): void
    {
        // Only applies to Articles
        if ($resourceClass !== Article::class) {
            return;
        }

        // Admin sees everything
        if ($this->security->isGranted('ROLE_ADMIN')) {
            return;
        }

        $user = $this->security->getUser();
        $rootAlias = $queryBuilder->getRootAliases()[0];

        // Automatic filter by author
        $queryBuilder
            ->andWhere(sprintf('%s.author = :current_user', $rootAlias))
            ->setParameter('current_user', $user);
    }
}

// Global Doctrine filter to exclude deleted items
namespace App\Doctrine\Filter;

use Doctrine\ORM\Mapping\ClassMetadata;
use Doctrine\ORM\Query\Filter\SQLFilter;

class SoftDeleteFilter extends SQLFilter
{
    public function addFilterConstraint(ClassMetadata $targetEntity, string $targetTableAlias): string
    {
        // Check if entity has a deletedAt field
        if (!$targetEntity->hasField('deletedAt')) {
            return '';
        }

        return sprintf('%s.deleted_at IS NULL', $targetTableAlias);
    }
}

# config/packages/doctrine.yaml
doctrine:
    orm:
        filters:
            soft_delete:
                class: App\Doctrine\Filter\SoftDeleteFilter
                enabled: true

# config/packages/security.yaml
security:
    password_hashers:
        Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface: 'auto'

    providers:
        app_user_provider:
            entity:
                class: App\Entity\User
                property: email

    firewalls:
        dev:
            pattern: ^/(_(profiler|wdt)|css|images|js)/
            security: false

        api:
            pattern: ^/api
            stateless: true
            jwt: ~  # Lexik JWT Bundle

        main:
            lazy: true
            provider: app_user_provider
            form_login:
                login_path: app_login
                check_path: app_login
                enable_csrf: true
            logout:
                path: app_logout
            remember_me:
                secret: '%kernel.secret%'
                lifetime: 604800

    access_control:
        - { path: ^/api/login, roles: PUBLIC_ACCESS }
        - { path: ^/api, roles: ROLE_USER }
        - { path: ^/admin, roles: ROLE_ADMIN }

// Custom authenticator for specific logic
namespace App\Security;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
use Symfony\Component\Security\Core\Exception\AuthenticationException;
use Symfony\Component\Security\Http\Authenticator\AbstractAuthenticator;
use Symfony\Component\Security\Http\Authenticator\Passport\Badge\UserBadge;
use Symfony\Component\Security\Http\Authenticator\Passport\Passport;
use Symfony\Component\Security\Http\Authenticator\Passport\SelfValidatingPassport;

class ApiKeyAuthenticator extends AbstractAuthenticator
{
    public function supports(Request $request): ?bool
    {
        // This authenticator only handles requests with X-API-KEY
        return $request->headers->has('X-API-KEY');
    }

    public function authenticate(Request $request): Passport
    {
        $apiKey = $request->headers->get('X-API-KEY');

        if (null === $apiKey) {
            throw new AuthenticationException('No API key provided');
        }

        // UserBadge loads user by identifier
        return new SelfValidatingPassport(
            new UserBadge($apiKey, function (string $apiKey) {
                // Load user by API key logic
                return $this->userRepository->findByApiKey($apiKey);
            })
        );
    }

    public function onAuthenticationSuccess(Request $request, TokenInterface $token, string $firewallName): ?Response
    {
        // null = continue request normally
        return null;
    }

    public function onAuthenticationFailure(Request $request, AuthenticationException $exception): ?Response
    {
        return new JsonResponse([
            'error' => $exception->getMessage(),
        ], Response::HTTP_UNAUTHORIZED);
    }
}

namespace App\Security\Voter;

use App\Entity\Article;
use App\Entity\User;
use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
use Symfony\Component\Security\Core\Authorization\Voter\Voter;

class ArticleVoter extends Voter
{
    public const VIEW = 'ARTICLE_VIEW';
    public const EDIT = 'ARTICLE_EDIT';
    public const DELETE = 'ARTICLE_DELETE';

    protected function supports(string $attribute, mixed $subject): bool
    {
        // This voter only handles Articles and these attributes
        return in_array($attribute, [self::VIEW, self::EDIT, self::DELETE])
            && $subject instanceof Article;
    }

    protected function voteOnAttribute(string $attribute, mixed $subject, TokenInterface $token): bool
    {
        $user = $token->getUser();
        $article = $subject;

        // Published articles are visible to everyone
        if ($attribute === self::VIEW && $article->isPublished()) {
            return true;
        }

        // Other actions require an authenticated user
        if (!$user instanceof User) {
            return false;
        }

        return match ($attribute) {
            self::VIEW => $this->canView($article, $user),
            self::EDIT => $this->canEdit($article, $user),
            self::DELETE => $this->canDelete($article, $user),
            default => false,
        };
    }

    private function canView(Article $article, User $user): bool
    {
        // Drafts visible only to author or admins
        return $article->getAuthor() === $user
            || in_array('ROLE_ADMIN', $user->getRoles());
    }

    private function canEdit(Article $article, User $user): bool
    {
        // Only author can edit
        return $article->getAuthor() === $user;
    }

    private function canDelete(Article $article, User $user): bool
    {
        // Author or admin can delete
        return $article->getAuthor() === $user
            || in_array('ROLE_ADMIN', $user->getRoles());
    }
}

// Using Voter in a controller
use Symfony\Component\Security\Http\Attribute\IsGranted;

class ArticleController extends AbstractController
{
    #[Route('/articles/{id}/edit', name: 'article_edit')]
    #[IsGranted(ArticleVoter::EDIT, subject: 'article')]
    public function edit(Article $article): Response
    {
        // Authorization is checked automatically
        // 403 if voter denies access
    }

    // Programmatic alternative
    #[Route('/articles/{id}', name: 'article_show')]
    public function show(Article $article): Response
    {
        $this->denyAccessUnlessGranted(ArticleVoter::VIEW, $article);

        // Or with condition
        if (!$this->isGranted(ArticleVoter::EDIT, $article)) {
            // Hide edit button
        }
    }
}

{# In Twig #}
{% if is_granted('ARTICLE_EDIT', article) %}
    <a href="{{ path('article_edit', {id: article.id}) }}">Edit</a>
{% endif %}

# config/packages/lexik_jwt_authentication.yaml
lexik_jwt_authentication:
    secret_key: '%env(resolve:JWT_SECRET_KEY)%'
    public_key: '%env(resolve:JWT_PUBLIC_KEY)%'
    pass_phrase: '%env(JWT_PASSPHRASE)%'
    token_ttl: 3600  # 1 hour

namespace App\Controller\Api;

use App\Entity\User;
use Lexik\Bundle\JWTAuthenticationBundle\Services\JWTTokenManagerInterface;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\PasswordHasher\Hasher\UserPasswordHasherInterface;
use Symfony\Component\Routing\Attribute\Route;

#[Route('/api')]
class AuthController extends AbstractController
{
    #[Route('/login', name: 'api_login', methods: ['POST'])]
    public function login(
        Request $request,
        UserPasswordHasherInterface $passwordHasher,
        JWTTokenManagerInterface $jwtManager,
    ): JsonResponse {
        $data = json_decode($request->getContent(), true);

        $user = $this->userRepository->findOneBy(['email' => $data['email']]);

        if (!$user || !$passwordHasher->isPasswordValid($user, $data['password'])) {
            return new JsonResponse(['error' => 'Invalid credentials'], 401);
        }

        // Generate JWT token
        $token = $jwtManager->create($user);

        return new JsonResponse([
            'token' => $token,
            'user' => [
                'id' => $user->getId(),
                'email' => $user->getEmail(),
                'roles' => $user->getRoles(),
            ],
        ]);
    }

    #[Route('/refresh-token', name: 'api_refresh_token', methods: ['POST'])]
    public function refreshToken(): JsonResponse
    {
        // Handled automatically by bundle if configured
        // Returns a new token from refresh token
    }
}

// JWT payload customization
namespace App\EventListener;

use Lexik\Bundle\JWTAuthenticationBundle\Event\JWTCreatedEvent;

class JWTCreatedListener
{
    public function onJWTCreated(JWTCreatedEvent $event): void
    {
        $user = $event->getUser();
        $payload = $event->getData();

        // Add custom data to token
        $payload['user_id'] = $user->getId();
        $payload['email'] = $user->getEmail();
        $payload['permissions'] = $user->getPermissions();

        $event->setData($payload);
    }
}

namespace App\Form;

use App\Entity\Article;
use App\Entity\Category;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\DateTimeType;
use Symfony\Component\Form\Extension\Core\Type\FileType;
use Symfony\Component\Form\Extension\Core\Type\TextareaType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Validator\Constraints as Assert;

class ArticleType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder
            ->add('title', TextType::class, [
                'label' => 'Article Title',
                'attr' => ['placeholder' => 'Enter title...'],
                'constraints' => [
                    new Assert\NotBlank(message: 'Title is required'),
                    new Assert\Length(
                        min: 10,
                        max: 255,
                        minMessage: 'Title must be at least {{ limit }} characters',
                    ),
                ],
            ])
            ->add('content', TextareaType::class, [
                'label' => 'Content',
                'constraints' => [
                    new Assert\NotBlank(),
                    new Assert\Length(min: 100),
                ],
            ])
            ->add('category', EntityType::class, [
                'class' => Category::class,
                'choice_label' => 'name',
                'placeholder' => 'Choose a category',
                'query_builder' => function ($repo) {
                    return $repo->createQueryBuilder('c')
                        ->where('c.active = true')
                        ->orderBy('c.name', 'ASC');
                },
            ])
            ->add('coverImage', FileType::class, [
                'label' => 'Cover Image',
                'mapped' => false,  // Not bound to entity
                'required' => false,
                'constraints' => [
                    new Assert\Image(
                        maxSize: '5M',
                        mimeTypes: ['image/jpeg', 'image/png', 'image/webp'],
                        mimeTypesMessage: 'Unsupported image format',
                    ),
                ],
            ])
            ->add('publishedAt', DateTimeType::class, [
                'widget' => 'single_text',
                'required' => false,
                'label' => 'Publication Date',
            ]);

        // Event listener to modify form dynamically
        $builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) {
            $article = $event->getData();
            $form = $event->getForm();

            // Add field only for modifications
            if ($article && $article->getId()) {
                $form->add('slug', TextType::class, [
                    'disabled' => true,
                    'help' => 'Slug cannot be modified',
                ]);
            }
        });
    }

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefaults([
            'data_class' => Article::class,
            'validation_groups' => ['Default', 'article_creation'],
        ]);
    }
}

#[Route('/articles/new', name: 'article_new')]
public function new(Request $request, SluggerInterface $slugger): Response
{
    $article = new Article();
    $form = $this->createForm(ArticleType::class, $article);
    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        // Handle file upload
        $coverImage = $form->get('coverImage')->getData();
        if ($coverImage) {
            $filename = $slugger->slug($article->getTitle()).'-'.uniqid().'.'.$coverImage->guessExtension();
            $coverImage->move($this->getParameter('covers_directory'), $filename);
            $article->setCoverImageFilename($filename);
        }

        $this->entityManager->persist($article);
        $this->entityManager->flush();

        $this->addFlash('success', 'Article created successfully!');
        return $this->redirectToRoute('article_show', ['id' => $article->getId()]);
    }

    return $this->render('article/new.html.twig', [
        'form' => $form,
    ]);
}

// Custom constraint
namespace App\Validator;

use Symfony\Component\Validator\Constraint;

#[\Attribute(\Attribute::TARGET_PROPERTY)]
class UniqueEmail extends Constraint
{
    public string $message = 'The email "{{ value }}" is already used.';
    public ?int $excludeId = null;  // To exclude current user in update
}

// Validator associated with the constraint
namespace App\Validator;

use App\Repository\UserRepository;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
use Symfony\Component\Validator\Exception\UnexpectedTypeException;

class UniqueEmailValidator extends ConstraintValidator
{
    public function __construct(
        private readonly UserRepository $userRepository,
    ) {}

    public function validate(mixed $value, Constraint $constraint): void
    {
        if (!$constraint instanceof UniqueEmail) {
            throw new UnexpectedTypeException($constraint, UniqueEmail::class);
        }

        if (null === $value || '' === $value) {
            return; // Let NotBlank handle empty values
        }

        $existingUser = $this->userRepository->findOneBy(['email' => $value]);

        // Check if user exists with this email
        // and it's not the current user (for updates)
        if ($existingUser && $existingUser->getId() !== $constraint->excludeId) {
            $this->context->buildViolation($constraint->message)
                ->setParameter('{{ value }}', $value)
                ->addViolation();
        }
    }
}

// Using constraint on entity
use App\Validator as AppAssert;
use Symfony\Component\Validator\Constraints as Assert;

class User
{
    #[Assert\NotBlank]
    #[Assert\Email]
    #[AppAssert\UniqueEmail]
    private ?string $email = null;

    #[Assert\NotBlank]
    #[Assert\Length(min: 8)]
    #[Assert\Regex(
        pattern: '/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)/',
        message: 'Password must contain an uppercase, lowercase, and digit'
    )]
    private ?string $plainPassword = null;
}

// Class-level constraint for multi-field validation
// src/Validator/PasswordMatch.php
#[\Attribute(\Attribute::TARGET_CLASS)]
class PasswordMatch extends Constraint
{
    public string $message = 'Passwords do not match.';

    public function getTargets(): string
    {
        return self::CLASS_CONSTRAINT;
    }
}

// The message (DTO containing data)
namespace App\Message;

final class SendWelcomeEmail
{
    public function __construct(
        public readonly int $userId,
        public readonly string $locale = 'en',
    ) {}
}

// The handler that processes the message
namespace App\MessageHandler;

use App\Message\SendWelcomeEmail;
use App\Repository\UserRepository;
use Symfony\Bridge\Twig\Mime\TemplatedEmail;
use Symfony\Component\Mailer\MailerInterface;
use Symfony\Component\Messenger\Attribute\AsMessageHandler;

#[AsMessageHandler]
final class SendWelcomeEmailHandler
{
    public function __construct(
        private readonly UserRepository $userRepository,
        private readonly MailerInterface $mailer,
    ) {}

    public function __invoke(SendWelcomeEmail $message): void
    {
        $user = $this->userRepository->find($message->userId);

        if (!$user) {
            return; // User deleted in the meantime
        }

        $email = (new TemplatedEmail())
            ->to($user->getEmail())
            ->subject('Welcome to our platform!')
            ->htmlTemplate('emails/welcome.html.twig')
            ->context([
                'user' => $user,
                'locale' => $message->locale,
            ]);

        $this->mailer->send($email);
    }
}

# config/packages/messenger.yaml
framework:
    messenger:
        failure_transport: failed

        transports:
            async:
                dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
                retry_strategy:
                    max_retries: 3
                    delay: 1000
                    multiplier: 2
            failed:
                dsn: 'doctrine://default?queue_name=failed'

        routing:
            # Route messages to async transport
            App\Message\SendWelcomeEmail: async
            App\Message\ProcessImage: async
            App\Message\GenerateReport: async

// Dispatching the message
use Symfony\Component\Messenger\MessageBusInterface;

class RegistrationController extends AbstractController
{
    #[Route('/register', name: 'app_register', methods: ['POST'])]
    public function register(
        Request $request,
        MessageBusInterface $bus,
    ): Response {
        // ... user creation

        // Async dispatch - email will be sent in background
        $bus->dispatch(new SendWelcomeEmail(
            userId: $user->getId(),
            locale: $request->getLocale(),
        ));

        // Immediate response to user
        return $this->redirectToRoute('app_login');
    }
}

namespace App\MessageHandler;

use App\Exception\PaymentFailedException;
use App\Exception\PaymentRetryableException;
use App\Message\ProcessPayment;
use Symfony\Component\Messenger\Attribute\AsMessageHandler;
use Symfony\Component\Messenger\Exception\RecoverableMessageHandlingException;
use Symfony\Component\Messenger\Exception\UnrecoverableMessageHandlingException;

#[AsMessageHandler]
final class ProcessPaymentHandler
{
    public function __invoke(ProcessPayment $message): void
    {
        try {
            $this->paymentGateway->process($message->orderId);
        } catch (PaymentRetryableException $e) {
            // Temporary error (timeout, rate limit) → retry
            throw new RecoverableMessageHandlingException(
                $e->getMessage(),
                previous: $e
            );
        } catch (PaymentFailedException $e) {
            // Permanent error (invalid card) → no retry
            throw new UnrecoverableMessageHandlingException(
                $e->getMessage(),
                previous: $e
            );
        }
    }
}

// Message-level retry configuration
namespace App\Message;

use Symfony\Component\Messenger\Stamp\DelayStamp;

final class ProcessPayment
{
    public function __construct(
        public readonly int $orderId,
        public readonly int $attempt = 1,
    ) {}

    // Custom retry delay based on attempt count
    public function getRetryDelay(): int
    {
        return match ($this->attempt) {
            1 => 5000,   // 5 seconds
            2 => 30000,  // 30 seconds
            3 => 300000, // 5 minutes
            default => 600000,
        };
    }
}

# Failed message management commands
php bin/console messenger:failed:show          # List failed messages
php bin/console messenger:failed:retry         # Retry all messages
php bin/console messenger:failed:retry 123     # Retry specific message
php bin/console messenger:failed:remove 123    # Remove a message

// Unit test: tests an isolated class
namespace App\Tests\Unit\Service;

use App\Service\PriceCalculator;
use PHPUnit\Framework\TestCase;

class PriceCalculatorTest extends TestCase
{
    private PriceCalculator $calculator;

    protected function setUp(): void
    {
        $this->calculator = new PriceCalculator();
    }

    public function testCalculateTotalWithoutDiscount(): void
    {
        $total = $this->calculator->calculateTotal(100.00, 0);

        $this->assertEquals(100.00, $total);
    }

    public function testCalculateTotalWithPercentageDiscount(): void
    {
        $total = $this->calculator->calculateTotal(100.00, 20);

        $this->assertEquals(80.00, $total);
    }

    /**
     * @dataProvider discountProvider
     */
    public function testCalculateTotalWithVariousDiscounts(
        float $price,
        int $discount,
        float $expected
    ): void {
        $total = $this->calculator->calculateTotal($price, $discount);

        $this->assertEquals($expected, $total);
    }

    public static function discountProvider(): array
    {
        return [
            'no discount' => [100.00, 0, 100.00],
            '10% discount' => [100.00, 10, 90.00],
            '50% discount' => [200.00, 50, 100.00],
            'max discount' => [100.00, 100, 0.00],
        ];
    }
}