As a PHP developer, writing clean and scalable code is crucial for building maintainable applications. One of the best ways to achieve this is by following the SOLID principles—five key design principles that improve the structure and flexibility of your code. In this blog, I'll walk through each SOLID principle, provide simple examples, and share my personal opinion on why they matter.

I don't want to be a teacher here, but since I will finish uni in June 2025, I think I can give my opinion. I also use these principles when designing new systems or refactoring them.

What Are the SOLID Principles?

SOLID is an acronym for five fundamental object-oriented design principles:

Single Responsibility Principle (SRP)
Open/Closed Principle (OCP)
Liskov Substitution Principle (LSP)
Interface Segregation Principle (ISP)
Dependency Inversion Principle (DIP)

By following these principles, we can create software that is easier to maintain, extend, and test.

1. Single Responsibility Principle (SRP)

"A class should have only one reason to change."

Each class should only be responsible for a single functionality. This keeps our code modular and easier to manage.

Example:

class ReportGenerator {
    public function generateReport() {
        // Generates a report
    }
}

class ReportSaver {
    public function saveToFile($report) {
        // Saves report to file
    }
}

My Opinion: This principle is crucial because it prevents bloated classes. It makes debugging and extending the code much easier.

2. Open/Closed Principle (OCP)

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

You should be able to extend a class’s behavior without modifying its existing code.

Example:

interface PaymentMethod {
    public function pay($amount);
}

class PayPalPayment implements PaymentMethod {
    public function pay($amount) {
        echo "Paying $amount via PayPal";
    }
}

class CreditCardPayment implements PaymentMethod {
    public function pay($amount) {
        echo "Paying $amount via Credit Card";
    }
}

My Opinion: This principle helps in keeping the core logic intact while allowing flexibility. It’s a lifesaver when working with large applications.

3. Liskov Substitution Principle (LSP)

"Subtypes must be substitutable for their base types without altering the correctness of the program."

Example:

class Bird {
    public function fly() {
        return "I can fly";
    }
}

class Penguin extends Bird {
    public function fly() {
        return "I can't fly"; // Violates LSP
    }
}

My Opinion: This principle ensures that child classes don’t break the behavior of parent classes. It avoids unexpected bugs.

4. Interface Segregation Principle (ISP)

"A class should not be forced to implement interfaces it does not use."

Example:

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

class HumanWorker implements Worker {
    public function work() {
        return "I am working";
    }
    public function eat() {
        return "I am eating";
    }
}

class RobotWorker implements Worker {
    public function work() {
        return "I am working";
    }
    public function eat() {
        // Robots don't eat, violates ISP
    }
}

Better Approach:

interface Workable {
    public function work();
}

interface Eatable {
    public function eat();
}

class RobotWorker implements Workable {
    public function work() {
        return "I am working";
    }
}

My Opinion: I find this principle extremely useful when designing large systems with many dependencies.

5. Dependency Inversion Principle (DIP)

"Depend on abstractions, not on concrete implementations."

Example:

class MySQLConnection {
    public function connect() {
        return "Connected to MySQL";
    }
}

class UserRepository {
    private $db;
    public function __construct(MySQLConnection $db) {
        $this->db = $db;
    }
}

Better Approach:

interface DatabaseConnection {
    public function connect();
}

class MySQLConnection implements DatabaseConnection {
    public function connect() {
        return "Connected to MySQL";
    }
}

class UserRepository {
    private $db;
    public function __construct(DatabaseConnection $db) {
        $this->db = $db;
    }
}

My Opinion: Using dependency inversion makes code easier to test and scale. It’s a must-have in professional applications.

Final Thoughts

The SOLID principles are essential for writing clean, maintainable, and scalable PHP applications. While they might seem overwhelming at first, once you start applying them, you’ll see a huge improvement in your code quality.

Personally, I believe that these principles are not strict rules but guidelines that help us write better software. Understanding when and how to apply them is what truly makes a great developer.

Understanding SOLID Principles in PHP: Writing Clean and Maintainable Code