SOLID Principles in PHP

Introduction

As software projects grow, writing clean and maintainable code becomes very important. If code is not properly structured, it becomes difficult to modify, debug, or extend the application later.

This is where SOLID principles help developers write better Object-Oriented code.

SOLID is a set of five design principles that help developers create software that is easier to maintain, extend, and understand. These principles are widely used in modern PHP applications, frameworks, and large-scale projects.

Learning SOLID principles will help you design classes and systems that follow good software architecture practices.

What are SOLID Principles?

SOLID is a set of 5 object-oriented design principles that help you write:

• Clean code
• Maintainable code
• Scalable applications

SOLID stands for:

1. S – Single Responsibility Principle
2. O – Open/Closed Principle
3. L – Liskov Substitution Principle
4. I – Interface Segregation Principle
5. D – Dependency Inversion Principle

Don’t worry — we’ll go one by one 😊

Single Responsibility Principle (SRP)

Rule:

A class should have only one reason to change.

This means a class should perform only one specific task.

Bad Example

class User {
    public function save() {}
    public function sendEmail() {}
}

Two responsibilities:

• Data handling
• Email sending

Bad Example

class User {
    public function saveUser() {
        // save user to database
    }

    public function sendEmail() {
        // send email to user
    }
}

Here, the class is doing two different tasks.

Good Example

class User {
    public function save() {}
}

class EmailService {
    public function send() {}
}

✔ One class → one responsibility

2. Open/Closed Principle (OCP)

Rule:

Open for extension, closed for modification.

This means you should be able to add new functionality without modifying existing code.

Bad Example

class Payment {
    public function pay($type) {
        if ($type == 'card') {}
        if ($type == 'upi') {}
    }
}

Good Example

interface Payment {
    public function pay();
}

class CardPayment implements Payment {
    public function pay() {}
}

class UpiPayment implements Payment {
    public function pay() {}
}

✔ New payment? Add class, don’t edit existing code.

3. Liskov Substitution Principle (LSP)

Rule:

Child class should be usable in place of parent class without breaking functionality.

In simple words, a subclass should behave correctly when used in place of its parent class.

Bad Example

class Bird {
    public function fly() {}
}

class Penguin extends Bird {
    public function fly() {
        throw new Exception("Can't fly");
    }
}

❌ Breaks expectation.

Good Design

interface Flyable {
    public function fly();
}

class Sparrow implements Flyable {
    public function fly() {}
}

✔ Correct behavior guaranteed.

4. Interface Segregation Principle (ISP)

Rule:

Don’t force a class to implement methods it doesn’t need.

Instead of creating one large interface, it is better to create multiple small interfaces.

Bad Example

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

Good Example

interface Workable {
    public function work();
}

interface Eatable {
    public function eat();
}

✔ Small, focused interfaces.

5. Dependency Inversion Principle (DIP)

Rule:

High-level modules should depend on abstractions, not concrete classes.

Instead of depending on concrete classes, your code should depend on interfaces or abstract classes.

Bad Example

class Order {
    public function __construct() {
        $this->payment = new CardPayment();
    }
}

Good Example

class Order {
    private $payment;

    public function __construct(Payment $payment) {
        $this->payment = $payment;
    }
}

✔ Flexible
✔ Testable
✔ Scalable

Why SOLID Matters in Real Projects?

• SOLID principles help developers:
• Write clean and organized code
• Reduce code complexity
• Make applications easier to maintain
• Improve code reusability
• Make systems easier to extend
• Avoid breaking existing functionality when adding new features
• These principles are commonly used in large PHP applications and frameworks.

Laravel, Symfony — all follow SOLID.

Don’t Memorize, Understand

You don’t apply SOLID everywhere.
You recognize when it’s needed.

That’s mastery.

Practical Task

Try creating a small payment system in PHP.

1. Create an interface called PaymentMethod.
2. Add a method pay().
3. Create two classes:
   • CardPayment
   • UpiPayment
4. Implement the pay() method in both classes.

Observe how each class follows the same structure but different behavior.

Summary

SOLID principles are a set of five design guidelines used in Object-Oriented Programming.

They help developers write code that is:

• Clean
• Maintainable
• Scalable
• Flexible

The five SOLID principles are:

• Single Responsibility Principle
• Open/Closed Principle
• Liskov Substitution Principle
• Interface Segregation Principle
• Dependency Inversion Principle

Learning SOLID principles helps you build professional-quality PHP applications.

In the next tutorial, we’ll see a project for login and registration based on OOPs concept.