SOLID Principles Knowledge Base

Overview

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

Principle	Name	Core Idea
S	Single Responsibility	One class = one reason to change
O	Open/Closed	Open for extension, closed for modification
L	Liskov Substitution	Subtypes must be substitutable for base types
I	Interface Segregation	Many specific interfaces > one general
D	Dependency Inversion	Depend on abstractions, not concretions

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	DDD Application
SRP	Aggregates have single consistency boundary
OCP	Domain Events enable extension without modification
LSP	Value Objects are substitutable (same type = same behavior)
ISP	Repository interfaces segregated (Reader/Writer)
DIP	Domain depends on Repository interfaces, not implementations

SOLID & Clean Architecture

Layer	SOLID Focus
Domain	SRP (Entities), LSP (Value Objects), ISP (Repository interfaces)
Application	SRP (Use Cases), DIP (Port interfaces)
Infrastructure	OCP (Adapters), DIP (Implements ports)
Presentation	SRP (Controllers), ISP (API contracts)

Severity Levels

Level	Description	Example
CRITICAL	Fundamental violation affecting entire system	God class, no DI
WARNING	Localized violation, should be fixed	instanceof chains
INFO	Minor issue, consider refactoring	Interface with 6 methods

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.