When striving to write clean, maintainable, and scalable PHP code, few concepts are as universally endorsed as the SOLID principles. Originally introduced by Robert C. Martin (Uncle Bob), SOLID represents five object-oriented design guidelines that help developers produce robust software architectures. These principles are especially relevant for PHP developers working with modern frameworks or building long-term applications.

In this post, we’ll break down each of the five principles in the SOLID acronym, illustrate them with PHP code, and discuss their benefits in real-world development. Whether you’re just starting with PHP or looking to refine your architectural skills, understanding SOLID is a must.

“Clean code always looks like it was written by someone who cares.”
— Robert C. Martin

S – Single Responsibility Principle (SRP)

A class should have only one reason to change.

In PHP, it’s easy to cram responsibilities into a single class — especially in controllers or services. But doing so makes code harder to maintain and test.

Bad Example:

class InvoiceManager {
    public function createInvoice(array $data) {
        // Validate input
        // Save invoice to database
        // Send confirmation email
    }
}
PHP

Better Approach (SRP Applied):

class InvoiceValidator {
    public function validate(array $data): bool {
        // Validation logic
    }
}

class InvoiceRepository {
    public function save(array $data): void {
        // DB logic
    }
}

class EmailNotifier {
    public function sendConfirmation(array $invoice): void {
        // Email logic
    }
}

class InvoiceManager {
    public function __construct(
        private InvoiceValidator $validator,
        private InvoiceRepository $repository,
        private EmailNotifier $notifier
    ) {}

    public function createInvoice(array $data) {
        if ($this->validator->validate($data)) {
            $this->repository->save($data);
            $this->notifier->sendConfirmation($data);
        }
    }
}
PHP

By giving each class a single focus, your code becomes easier to extend and less fragile when requirements change.

O – Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

Instead of editing existing classes to add new behavior, we should extend them.

Violating OCP:

class DiscountCalculator {
    public function calculate(Order $order) {
        if ($order->type === 'percentage') {
            return $order->total * 0.9;
        }

        if ($order->type === 'fixed') {
            return $order->total * 0.8;
        }

        return $order->total;
    }
}
PHP

OCP-Compliant Design:

interface DiscountStrategy {
    public function apply(float $amount): float;
}

class PercentageDiscount implements DiscountStrategy {
    public function apply(float $amount): float {
        return $amount * 0.9;
    }
}

class FixedDiscount implements DiscountStrategy {
    public function apply(float $amount): float {
        return $amount - 10;
    }
}

class DiscountCalculator {
    public function __construct(private DiscountStrategy $strategy) {}

    public function calculate(float $amount): float {
        return $this->strategy->apply($amount);
    }
}
PHP

Adding a new discount type now only requires creating a new class that implements DiscountStrategy.

L – Liskov Substitution Principle (LSP)

Objects of a superclass should be replaceable with objects of its subclasses without breaking the application.

Violating LSP often happens when a subclass overrides behavior in a way that contradicts the expectations set by the base class.

LSP Violation Example:

class Bird {
    public function fly() {
        // flying logic
    }
}

class Ostrich extends Bird {
    public function fly() {
        throw new \Exception("Ostriches can't fly!");
    }
}
PHP

This design will break wherever we assume that all birds can fly.

LSP-Compliant Design:

interface Bird {
    public function move();
}

class Sparrow implements Bird {
    public function move() {
        // fly
    }
}

class Ostrich implements Bird {
    public function move() {
        // walk
    }
}
PHP

Now, both birds implement move, but their behavior aligns with their capabilities, preserving substitutability.

I – Interface Segregation Principle (ISP)

Clients should not be forced to depend on methods they do not use.

In PHP, fat interfaces can force classes to implement unnecessary methods.

Bad Interface:

interface Worker {
    public function work();
    public function eat();
}
PHP

Problem: What if a Robot is a worker but doesn’t eat?

ISP Solution:

interface Workable {
    public function work();
}

interface Eatable {
    public function eat();
}

class Human implements Workable, Eatable {
    public function work() { /* ... */ }
    public function eat() { /* ... */ }
}

class Robot implements Workable {
    public function work() { /* ... */ }
}
PHP

This separation allows developers to implement only what’s relevant for each class.

D – Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Instead of hard-coding dependencies, depend on interfaces.

Tightly Coupled Code:

class PasswordReminder {
    private $db;

    public function __construct() {
        $this->db = new MySQLConnection();
    }
}
PHP

DIP-Friendly Refactor:

interface DBConnectionInterface {
    public function connect();
}

class MySQLConnection implements DBConnectionInterface {
    public function connect() { /* ... */ }
}

class PasswordReminder {
    public function __construct(private DBConnectionInterface $db) {}
}
PHP

Now you can swap out the implementation for testing or for a different database engine.

Final Thoughts: Why SOLID Matters in PHP

While SOLID principles originated in statically typed languages like Java and C#, they apply beautifully to PHP’s object-oriented features. Especially when working on large-scale Symfony or Laravel applications, following SOLID helps ensure:

  • Easier testing
  • Reduced code duplication
  • Better scalability
  • Stronger architectural foundations

The beauty of SOLID is that it's not a rigid rulebook — it's a guide to help you think more clearly about structure and responsibility in your code.