Writing code that is clear, scalable, and maintainable is more important than simply creating something that functions; it's also about developing something that will continue to function well as your application expands.
Bad coding habits in PHP development can result in a complex web of dependencies that makes it more difficult to debug, expand, or restructure your code.
The SOLID principles apply in this situation. Writing adaptable, reusable, and durable code that endures is made easier with the help of these five fundamental object-oriented design concepts.
You may make your application simpler to maintain and grow by following SOLID, which helps you avoid typical mistakes like tight coupling, code duplication, and unanticipated side effects.
Each of the SOLID principles will be thoroughly examined in this tutorial, along with practical PHP examples and how they might improve your development process.
Comprehending SOLID will elevate your PHP abilities, regardless of your level of expertise. This is true for both novices who want to produce better code and seasoned developers who want to improve their methods.
Understanding SOLID Principles
Software development that is scalable and maintainable is supported by the five design principles known as the SOLID principles. They are important for object-oriented programming (OOP) and were first presented by Robert C. Martin (Uncle Bob). SOLID is an acronym that stands for:
- Single Responsibility Principle (SRP)
- Open/Closed Principle (OCP)
- Liskov Substitution Principle (LSP)
- Interface Segregation Principle (ISP)
- Dependency Inversion Principle (DIP)
You may improve reusability, simplify debugging, and decrease code complexity by adhering to these guidelines.
Let's now examine each of them individually using PHP examples and more thorough explanations.
Single Responsibility Principle (SRP)
Definition: A class should have only one reason to change.
A class that manages several tasks increases coupling and complicates maintenance. Rather, divide issues into distinct classes.
Why SRP is Important
- Simplifies debugging and testing.
- Enhances reusability of code.
- Reduces unintended side effects when modifying code.
Example (Violating SRP)
class UserManager {
public function saveUser($user) {
// Code to save user to database
}
public function sendEmail($user) {
// Code to send an email
}
}
Here, UserManager is responsible for both user persistence and email notifications, which are unrelated concerns.
Example (Following SRP)
class UserRepository {
public function saveUser($user) {
// Code to save user to database
}
}
class EmailService {
public function sendEmail($user) {
// Code to send an email
}
}
EmailService now manages email alerts, whereas UserRepository simply manages database interactions. Every class is responsible for one thing.
Open/Closed Principle (OCP)
Definition: A class should be open for extension but closed for modification.
Why OCP is Important
- Reduces changes to existing code, minimizing bugs.
- Allows adding new functionalities without modifying existing classes.
- Promotes use of polymorphism for scalability.
Example (Violating OCP)
class PaymentProcessor {
public function payWithPayPal($amount) {
// Process PayPal payment
}
public function payWithStripe($amount) {
// Process Stripe payment
}
}
Adding a new payment method requires modifying this class, which violates OCP.
Example (Following OCP)
interface PaymentMethod {
public function pay($amount);
}
class PayPalPayment implements PaymentMethod {
public function pay($amount) {
// Process PayPal payment
}
}
class StripePayment implements PaymentMethod {
public function pay($amount) {
// Process Stripe payment
}
}
class PaymentProcessor {
public function pay(PaymentMethod $paymentMethod, $amount) {
$paymentMethod->pay($amount);
}
}
Now, new payment methods can be added without modifying PaymentProcessor, keeping the code open for extension but closed for modification.
Liskov Substitution Principle (LSP)
Definition: Subtypes must be substitutable for their base types without altering the correctness of the program.
Why LSP is Important
Ensures that functionality is extended by derived classes without disrupting existing behavior.
Promotes abstraction through the use of interfaces.
Example (Violating LSP)
class Rectangle {
protected $width;
protected $height;
public function setWidth($width) {
$this->width = $width;
}
public function setHeight($height) {
$this->height = $height;
}
}
class Square extends Rectangle {
public function setWidth($width) {
$this->width = $this->height = $width;
}
}
Here, Square modifies Rectangle's behavior unexpectedly, violating LSP.
Example (Following LSP)
interface Shape {
public function area();
}
class Rectangle implements Shape {
protected $width;
protected $height;
public function __construct($width, $height) {
$this->width = $width;
$this->height = $height;
}
public function area() {
return $this->width * $this->height;
}
}
class Square implements Shape {
protected $side;
public function __construct($side) {
$this->side = $side;
}
public function area() {
return $this->side * $this->side;
}
}
Now, Rectangle and Square both follow LSP, allowing for correct substitutions.
Interface Segregation Principle (ISP)
Definition: A class should not be forced to implement interfaces it does not use.
Example (Following ISP)
interface Workable {
public function work();
}
interface Eatable {
public function eat();
}
class Developer implements Workable {
public function work() {
// Developer writes code
}
}
class OfficeWorker implements Workable, Eatable {
public function work() {
// Office worker works
}
public function eat() {
// Office worker eats lunch
}
}
Now, each class implements only the interfaces relevant to its behavior.
Dependency Inversion Principle (DIP)
Definition: Depend on abstractions, not on concretions.
Example (Following DIP)
interface Database {
public function connect();
}
class MySQLDatabase implements Database {
public function connect() {
// MySQL connection
}
}
class UserRepository {
private $db;
public function __construct(Database $db) {
$this->db = $db;
}
}
Now, UserRepository can work with any Database that implements the Database interface, adhering to DIP.
How SOLID Principles Work with Modern PHP Frameworks
SOLID principles are naturally encouraged by a lot of modern PHP frameworks. See how:
Laravel & SOLID
✅ Single Responsibility Principle → Laravel divides issues into Controllers, Services, and Repositories.
✅ Open/Closed Principle → Service providers and middleware add capabilities without changing the fundamental logic.
✅ Dependency Inversion Principle → Dependency injection is made simple by Laravel's Service Container.
Symfony & SOLID
✅ Interface Segregation → The modular architecture is enforced by Symfony's Event Listeners and Services.
✅ Liskov Substitution → The interface-based design of Symfony guarantees accurate replacements.
Best Practices for Testing SOLID Code
Incorporate unit tests and mocking frameworks to guarantee the dependability of your SOLID code.
Testing a Repository with PHPUnit
class UserRepositoryTest extends TestCase {
public function testUserCanBeSaved() {
$mockDb = $this->createMock(Database::class);
$userRepo = new UserRepository($mockDb);
$this->assertTrue($userRepo->saveUser(new User("John")));
}
}
Using mocks ensures we test only the repository logic and not the database connection itself.
Final Cheat Sheet: Quick Reference
| Principle | What It Means | Why It Matters |
|---|---|---|
| SRP (Single Responsibility Principle) | A class should have only one job or reason to change. | Simplifies debugging, improves maintainability, and keeps code clean. |
| OCP (Open/Closed Principle) | Code should be open for extension but closed for modification. | Enables adding new features without altering existing, stable code. |
| LSP (Liskov Substitution Principle) | Subtypes must be usable wherever their base types are expected. | Prevents unexpected behaviors and ensures consistency in code. |
| ISP (Interface Segregation Principle) | Don't force classes to implement methods they don’t use. | Keeps interfaces clean, focused, and easy to work with. |
| DIP (Dependency Inversion Principle) | Depend on abstractions, not concrete implementations. | Enhances flexibility, promotes modular design, and improves testability. |
Conclusion
Applying SOLID principles to PHP code makes it more scalable, modular, and clean.
It is easier to manage, grow, and debug your code if you maintain accurate inheritance, rely on abstractions, utilize focused interfaces, make sure classes have single responsibilities, and provide extension without change.
Following these best practices can help you reduce technical debt, future-proof your systems, and increase reusability.
PHP code that is efficient, flexible, and long-lasting may be written using SOLID principles, regardless of the size of the project or system you're working on.
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Top comments (1)
Nice breakdown of SOLID principles in the context of PHP.
I enjoyed reading it.