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SKILL.md

PSR Overview Knowledge

What is PSR?

PSR (PHP Standards Recommendations) are specifications published by the PHP Framework Interoperability Group (PHP-FIG). They establish common standards for PHP code to ensure interoperability between frameworks and libraries.

Accepted PSRs Summary

PSR Name Category Status
PSR-1 Basic Coding Standard Coding Style Accepted
PSR-3 Logger Interface Logging Accepted
PSR-4 Autoloader Autoloading Accepted
PSR-6 Caching Interface Caching Accepted
PSR-7 HTTP Message Interface HTTP Accepted
PSR-11 Container Interface DI Container Accepted
PSR-12 Extended Coding Style Coding Style Accepted
PSR-13 Hypermedia Links Hypermedia Accepted
PSR-14 Event Dispatcher Events Accepted
PSR-15 HTTP Handlers HTTP Accepted
PSR-16 Simple Cache Caching Accepted
PSR-17 HTTP Factories HTTP Accepted
PSR-18 HTTP Client HTTP Accepted
PSR-20 Clock Time Accepted

PSR Categories

Coding Style (PSR-1, PSR-12)

Standards for writing clean, consistent PHP code.

Aspect PSR-1 PSR-12
Scope Basic rules Extended formatting
File encoding UTF-8 without BOM Inherits PSR-1
Class names StudlyCaps Inherits PSR-1
Method names camelCase Inherits PSR-1
Indentation - 4 spaces
Line length - 120 chars soft limit
Keywords - Lowercase

Autoloading (PSR-4)

Standard for autoloading classes from file paths.

Namespace Prefix → Base Directory
App\             → src/

FQCN                          → File Path
App\Domain\User\Entity\User   → src/Domain/User/Entity/User.php

HTTP (PSR-7, PSR-15, PSR-17, PSR-18)

Standards for HTTP messages, handlers, factories, and clients.

PSR Purpose Key Interfaces
PSR-7 HTTP Messages RequestInterface, ResponseInterface, StreamInterface
PSR-15 HTTP Handlers MiddlewareInterface, RequestHandlerInterface
PSR-17 HTTP Factories RequestFactoryInterface, ResponseFactoryInterface
PSR-18 HTTP Client ClientInterface 
PSR-17 (Factory) → PSR-7 (Message) → PSR-15 (Handler) → PSR-7 (Response)
                                          ↓
                                   PSR-18 (Client)

Caching (PSR-6, PSR-16)

Standards for caching implementations.

Aspect PSR-6 PSR-16
Complexity Full-featured Simple
Key interfaces CacheItemPoolInterface, CacheItemInterface CacheInterface
Deferred saves Yes No
Use case Complex caching needs Simple get/set

Logging (PSR-3)

Standard for logging libraries.

interface LoggerInterface {
    public function emergency(string|\Stringable $message, array $context = []): void;
    public function alert(string|\Stringable $message, array $context = []): void;
    public function critical(string|\Stringable $message, array $context = []): void;
    public function error(string|\Stringable $message, array $context = []): void;
    public function warning(string|\Stringable $message, array $context = []): void;
    public function notice(string|\Stringable $message, array $context = []): void;
    public function info(string|\Stringable $message, array $context = []): void;
    public function debug(string|\Stringable $message, array $context = []): void;
    public function log(mixed $level, string|\Stringable $message, array $context = []): void;
}

DI Container (PSR-11)

Standard for dependency injection containers.

interface ContainerInterface {
    public function get(string $id): mixed;
    public function has(string $id): bool;
}

Events (PSR-14)

Standard for event dispatching.

interface EventDispatcherInterface {
    public function dispatch(object $event): object;
}

interface ListenerProviderInterface {
    public function getListenersForEvent(object $event): iterable;
}

interface StoppableEventInterface {
    public function isPropagationStopped(): bool;
}

Hypermedia (PSR-13)

Standard for hypermedia links (HATEOAS).

interface LinkInterface {
    public function getHref(): string;
    public function isTemplated(): bool;
    public function getRels(): array;
    public function getAttributes(): array;
}

Time (PSR-20)

Standard for clock abstraction.

interface ClockInterface {
    public function now(): DateTimeImmutable;
}

When to Use Each PSR

Decision Matrix

Need PSR
Code formatting PSR-1, PSR-12
Class autoloading PSR-4
Logging PSR-3
Simple caching (get/set) PSR-16
Complex caching (pools, tags) PSR-6
HTTP requests/responses PSR-7
HTTP middleware PSR-15
Creating HTTP objects PSR-17
HTTP client for external APIs PSR-18
Dependency injection PSR-11
Event system PSR-14
REST API with links PSR-13
Testing with time PSR-20

Common Combinations

Use Case PSRs
HTTP API PSR-7 + PSR-15 + PSR-17
HTTP Client PSR-7 + PSR-17 + PSR-18
Web Application PSR-1 + PSR-4 + PSR-12 + PSR-3 + PSR-11
CQRS/Event-Driven PSR-14 + PSR-3 + PSR-11
Microservice All of the above

PHP Package Implementations

PSR-3: Logger

Package Description
monolog/monolog De facto standard logger
psr/log Interface only

PSR-4: Autoloader

Package Description
Composer Built-in autoloader

PSR-6: Cache

Package Description
symfony/cache Full-featured cache
cache/filesystem-adapter File-based cache

PSR-7/PSR-17: HTTP

Package Description
guzzlehttp/psr7 Guzzle implementation
nyholm/psr7 Lightweight implementation
laminas/laminas-diactoros Laminas implementation

PSR-11: Container

Package Description
php-di/php-di Autowiring DI container
league/container Flexible container
pimple/pimple Simple container

PSR-14: Event Dispatcher

Package Description
symfony/event-dispatcher Symfony implementation
league/event League implementation

PSR-15: HTTP Handlers

Package Description
middlewares/utils Middleware utilities
relay/relay Simple dispatcher

PSR-18: HTTP Client

Package Description
guzzlehttp/guzzle Full HTTP client
symfony/http-client Symfony HTTP client

PSR-20: Clock

Package Description
psr/clock Interface only
symfony/clock Symfony implementation
lcobucci/clock Simple implementation

Composer Requirements

{
    "require": {
        "psr/log": "^3.0",
        "psr/cache": "^3.0",
        "psr/http-message": "^2.0",
        "psr/http-factory": "^1.0",
        "psr/http-client": "^1.0",
        "psr/container": "^2.0",
        "psr/event-dispatcher": "^1.0",
        "psr/link": "^2.0",
        "psr/clock": "^1.0",
        "psr/simple-cache": "^3.0"
    }
}

Integration with DDD

Layer Mapping

DDD Layer Relevant PSRs Note
Domain PSR-3, PSR-14, PSR-20 PSR interfaces are pure contracts — acceptable in Domain
Application PSR-3, PSR-11, PSR-14, PSR-20 Service orchestration layer
Infrastructure PSR-6, PSR-16, PSR-18 Implementation layer
Presentation PSR-7, PSR-15, PSR-17 HTTP layer

Important: PSR packages (psr/log, psr/clock, psr/event-dispatcher) contain only interfaces — no implementation code. They are PHP community standards equivalent to a standard library. Using PSR interfaces in Domain layer is acceptable and common practice. What is NOT acceptable in Domain: implementation packages like monolog/monolog, symfony/cache, guzzlehttp/guzzle.

Example: CQRS Application

<?php

declare(strict_types=1);

namespace App\Application\User\Handler;

use App\Application\User\Command\CreateUserCommand;
use App\Domain\User\Entity\User;
use App\Domain\User\Repository\UserRepositoryInterface;
use Psr\EventDispatcher\EventDispatcherInterface;  // PSR-14
use Psr\Log\LoggerInterface;                       // PSR-3
use Psr\Clock\ClockInterface;                      // PSR-20

final readonly class CreateUserHandler
{
    public function __construct(
        private UserRepositoryInterface $repository,
        private EventDispatcherInterface $eventDispatcher,
        private LoggerInterface $logger,
        private ClockInterface $clock,
    ) {
    }

    public function __invoke(CreateUserCommand $command): void
    {
        $this->logger->info('Creating user', ['email' => $command->email]);

        $user = User::create(
            $command->email,
            $command->name,
            $this->clock->now(),
        );

        $this->repository->save($user);

        foreach ($user->pullEvents() as $event) {
            $this->eventDispatcher->dispatch($event);
        }
    }
}

Compliance Checklist

PSR Required For Check
PSR-1 All PHP projects phpcs --standard=PSR1
PSR-12 All PHP projects phpcs --standard=PSR12
PSR-4 All PHP projects composer dump-autoload --strict
PSR-3 Projects with logging Implement LoggerInterface
PSR-6/16 Projects with caching Implement CacheInterface
PSR-7 HTTP APIs Use PSR-7 implementations
PSR-11 Projects with DI Implement ContainerInterface
PSR-14 Event-driven projects Implement EventDispatcherInterface
PSR-15 HTTP middleware Implement MiddlewareInterface
PSR-17 HTTP object creation Use factory interfaces
PSR-18 External API calls Implement ClientInterface
PSR-20 Time-sensitive code Implement ClockInterface

See Also

  • references/accepted-psrs.md - Detailed PSR descriptions
  • references/when-to-use.md - Decision matrix for PSR selection
  • references/compatibility.md - Inter-PSR relationships
  • references/php-fig-process.md - How PSRs are created
  • assets/psr-selection-guide.md - Selection guide template
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