SOLID Principles Knowledge Base

Overview

SOLID is a set of five design principles for writing maintainable, extensible software.

Quick Detection Patterns

SRP Violations

# God classes (>500 lines)
find . -name "*.php" -exec wc -l {} \; | awk '$1 > 500 {print}'

# Classes with "And" in name
grep -rn "class.*And[A-Z]" --include="*.php"

# Classes with >7 dependencies
grep -rn "public function __construct" --include="*.php" -A 20 | grep -E "private|readonly" | wc -l

# Multiple responsibility indicators
grep -rn "class.*Manager\|class.*Handler\|class.*Processor" --include="*.php"

Signs of SRP Violation:

• Class has >500 lines
• Class has >7 dependencies
• Class name contains "And", "Or", "Manager", "Handler"
• Multiple unrelated public methods
• Changes for multiple business reasons

OCP Violations

# Switch on type
grep -rn "switch.*instanceof\|switch.*::class" --include="*.php"

# Type checking conditionals
grep -rn "if.*instanceof\|elseif.*instanceof" --include="*.php"

# Hardcoded type maps
grep -rn "\[.*::class.*=>" --include="*.php"

Signs of OCP Violation:

• Switch statements on object types
• instanceof chains in conditionals
• Adding new types requires modifying existing code
• Hardcoded type-to-behavior mappings

LSP Violations

# Exception in overridden methods
grep -rn "throw.*NotImplemented\|throw.*NotSupported" --include="*.php"

# Empty overrides
grep -rn "public function.*\{[\s]*\}" --include="*.php"

# Type checks in child classes
grep -rn "if.*parent::" --include="*.php"

Signs of LSP Violation:

• Child class throws NotImplementedException
• Child class has empty method overrides
• Parent type check in child class
• Preconditions strengthened in subtype
• Postconditions weakened in subtype

ISP Violations

# Large interfaces (>5 methods)
grep -rn "interface\s" --include="*.php" -A 30 | grep -c "public function"

# Empty interface implementations
grep -rn "// TODO\|// not implemented\|// unused" --include="*.php"

Signs of ISP Violation:

• Interface has >5 methods
• Classes implement interfaces partially
• Unused methods return null/throw
• Interface name too generic ("Service", "Manager")

DIP Violations

# Direct instantiation in constructors
grep -rn "new\s\+[A-Z]" --include="*.php" | grep -v "Exception\|DateTime\|stdClass"

# Static method calls
grep -rn "::[a-z].*(" --include="*.php" | grep -v "self::\|static::\|parent::"

# Concrete class type hints (not interfaces)
grep -rn "function.*([A-Z][a-z]*[A-Z]" --include="*.php"

Signs of DIP Violation:

• new ConcreteClass() inside methods
• Static calls to concrete classes
• Type hints to concrete classes (not interfaces)
• No constructor injection

PHP 8.4 Patterns

SRP Compliant

<?php

declare(strict_types=1);

// BAD: Multiple responsibilities
final class UserService
{
    public function register(UserData $data): User { /* ... */ }
    public function sendEmail(User $user): void { /* ... */ }
    public function generateReport(User $user): Report { /* ... */ }
}

// GOOD: Single responsibility each
final readonly class RegisterUserHandler
{
    public function __construct(
        private UserRepository $users,
        private EventDispatcher $events,
    ) {}

    public function __invoke(RegisterUserCommand $command): UserId
    {
        $user = User::register($command->email, $command->password);
        $this->users->save($user);
        $this->events->dispatch($user->releaseEvents());

        return $user->id;
    }
}

OCP Compliant

<?php

declare(strict_types=1);

// BAD: Modification required for new types
final class PaymentProcessor
{
    public function process(Payment $payment): void
    {
        match ($payment->type) {
            'card' => $this->processCard($payment),
            'paypal' => $this->processPaypal($payment),
            // Must modify for new payment types
        };
    }
}

// GOOD: Extension without modification
interface PaymentGateway
{
    public function supports(Payment $payment): bool;
    public function process(Payment $payment): PaymentResult;
}

final readonly class PaymentProcessor
{
    /** @param iterable<PaymentGateway> $gateways */
    public function __construct(
        private iterable $gateways,
    ) {}

    public function process(Payment $payment): PaymentResult
    {
        foreach ($this->gateways as $gateway) {
            if ($gateway->supports($payment)) {
                return $gateway->process($payment);
            }
        }
        throw new UnsupportedPaymentException($payment->type);
    }
}

LSP Compliant

<?php

declare(strict_types=1);

// BAD: Violates substitutability
abstract class Bird
{
    abstract public function fly(): void;
}

final class Penguin extends Bird
{
    public function fly(): void
    {
        throw new CannotFlyException(); // LSP violation!
    }
}

// GOOD: Proper abstraction hierarchy
interface Bird
{
    public function move(): void;
}

interface FlyingBird extends Bird
{
    public function fly(): void;
}

final readonly class Penguin implements Bird
{
    public function move(): void
    {
        $this->swim();
    }

    private function swim(): void { /* ... */ }
}

final readonly class Eagle implements FlyingBird
{
    public function move(): void
    {
        $this->fly();
    }

    public function fly(): void { /* ... */ }
}

ISP Compliant

<?php

declare(strict_types=1);

// BAD: Fat interface
interface UserRepository
{
    public function find(UserId $id): ?User;
    public function findByEmail(Email $email): ?User;
    public function save(User $user): void;
    public function delete(User $user): void;
    public function findAll(): array;
    public function count(): int;
    public function export(): string;
    public function import(string $data): void;
}

// GOOD: Segregated interfaces
interface UserReader
{
    public function find(UserId $id): ?User;
    public function findByEmail(Email $email): ?User;
}

interface UserWriter
{
    public function save(User $user): void;
    public function delete(User $user): void;
}

interface UserExporter
{
    public function export(): string;
    public function import(string $data): void;
}

// Compose as needed
interface UserRepository extends UserReader, UserWriter {}

DIP Compliant

<?php

declare(strict_types=1);

// BAD: Depends on concretions
final class OrderService
{
    public function process(Order $order): void
    {
        $mailer = new SmtpMailer();           // Concrete dependency
        $logger = Logger::getInstance();       // Static dependency
        $validator = new OrderValidator();     // Hidden dependency

        // ...
    }
}

// GOOD: Depends on abstractions
final readonly class OrderService
{
    public function __construct(
        private OrderRepository $orders,
        private Mailer $mailer,
        private LoggerInterface $logger,
        private OrderValidator $validator,
    ) {}

    public function process(Order $order): void
    {
        $this->validator->validate($order);
        $this->orders->save($order);
        $this->mailer->send(new OrderConfirmation($order));
        $this->logger->info('Order processed', ['id' => $order->id->value]);
    }
}

SOLID & DDD Integration

SOLID & Clean Architecture

Severity Levels

References

See detailed documentation in references/:

• srp-patterns.md - Single Responsibility patterns
• ocp-patterns.md - Open/Closed patterns
• lsp-patterns.md - Liskov Substitution patterns
• isp-patterns.md - Interface Segregation patterns
• dip-patterns.md - Dependency Inversion patterns
• antipatterns.md - Common SOLID violations

See assets/report-template.md for audit report format.