A Comprehensive Guide to PHP Object-Oriented Programming (OOP) Design Patterns with code example!

Introduction

In the rapidly evolving world of web development, writing clean, maintainable, and scalable code is more critical than ever. PHP, one of the most popular server-side scripting languages, provides robust support for Object-Oriented Programming (OOP). One of the key benefits of using OOP in PHP is the ability to implement design patterns—reusable solutions to common software design problems. This comprehensive guide will explore PHP OOP design patterns, providing insights into how they can help you write better, more efficient code.

What are Design Patterns?

Design patterns are well-proven solutions to common problems in software design. They represent best practices that have evolved over time and have been widely adopted by developers to solve recurring problems in software architecture. By using design patterns, developers can avoid reinventing the wheel and instead leverage these pre-established solutions to create more flexible and scalable applications.

In PHP, design patterns can be implemented using OOP principles such as encapsulation, inheritance, and polymorphism. These patterns can be categorized into three main types: Creational, Structural, and Behavioral.

To start off, here’s a list of common PHP Object-Oriented Programming (OOP) design patterns, categorized by their purpose:

1. Creational Design Patterns

• Singleton: Ensures that a class has only one instance and provides a global point of access to it.
• Factory Method: Provides an interface for creating objects in a superclass but allows subclasses to alter the type of objects that will be created.
• Abstract Factory: Provides an interface for creating families of related or dependent objects without specifying their concrete classes.
• Builder: Separates the construction of a complex object from its representation, allowing the same construction process to create different representations.
• Prototype: Allows cloning of existing objects without being dependent on their classes.
• Lazy Initialization: Delays the initialization of an object until it’s actually needed, often used in conjunction with other patterns like Singleton.

2. Structural Design Patterns

• Adapter: Allows objects with incompatible interfaces to work together by providing a wrapper that translates one interface to another.
• Bridge: Decouples an abstraction from its implementation, allowing them to vary independently.
• Composite: Composes objects into tree structures to represent part-whole hierarchies, allowing clients to treat individual objects and compositions of objects uniformly.
• Decorator: Allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class.
• Facade: Provides a simplified interface to a complex subsystem, making it easier to interact with.
• Flyweight: Reduces the cost of creating and managing a large number of similar objects by sharing objects that are identical in some way.
• Proxy: Provides a surrogate or placeholder for another object to control access to it.

3. Behavioral Design Patterns

• Chain of Responsibility: Passes a request along a chain of handlers, allowing multiple objects to handle the request without the sender needing to know which object will handle it.
• Command: Encapsulates a request as an object, thereby allowing for parameterization of clients with different requests, queuing of requests, and logging of requests.
• Interpreter: Implements a specialized language or grammar, and evaluates sentences in that language.
• Iterator: Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
• Mediator: Reduces the complexity of communication between multiple objects or classes by having them communicate through a central mediator object.
• Memento: Captures and externalizes an object’s internal state so that the object can be restored to this state later.
• Observer: Defines a one-to-many dependency between objects, where one object changes state, and all its dependents are notified and updated automatically.
• State: Allows an object to alter its behavior when its internal state changes, appearing as if the object changed its class.
• Strategy: Defines a family of algorithms, encapsulates each one, and makes them interchangeable.
• Template Method: Defines the skeleton of an algorithm in a method, deferring some steps to subclasses.
• Visitor: Allows you to add further operations to objects without having to modify them.

4. Other Patterns

• Dependency Injection: Provides a way to supply dependencies to an object at runtime, promoting loose coupling between components.
• MVC (Model-View-Controller): Separates an application into three interconnected components: Model (data), View (UI), and Controller (logic), allowing for separation of concerns.
• Service Locator: Provides a centralized point for managing and resolving services and their dependencies.
• Repository: Encapsulates the logic required to access data sources, allowing the business logic to interact with data sources through a common interface.
• Data Mapper: Transfers data between objects and a database while keeping them independent from each other.
• Active Record: An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data.

These design patterns provide proven solutions to common problems encountered in software design, promoting code that is more modular, reusable, and easier to maintain.

“I’ll provide you with some code examples for each category so you can get a clear understanding of what they entail. Enjoy!”.

Creational Design Patterns

Creational design patterns focus on the efficient and flexible creation of objects. These patterns provide various object creation mechanisms, which increase the flexibility and reuse of existing code.

1. Singleton Pattern

The Singleton pattern ensures that a class has only one instance and provides a global point of access to it. This pattern is particularly useful when managing resources such as database connections, where it’s essential to limit the number of instances.

class Database {
    private static $instance = null;
    private $connection;

    private function __construct() {
        $this->connection = new mysqli('localhost', 'user', 'password', 'database');
    }

    public static function getInstance() {
        if (self::$instance == null) {
            self::$instance = new Database();
        }
        return self::$instance;
    }

    public function getConnection() {
        return $this->connection;
    }
}

In this example, the Database class can only be instantiated once, and the same instance is reused throughout the application.

2. Factory Method Pattern

The Factory Method pattern defines an interface for creating an object but allows subclasses to alter the type of objects that will be created. This pattern is useful when the exact type of object is not known until runtime.

abstract class Product {
    abstract public function getType();
}

class ConcreteProductA extends Product {
    public function getType() {
        return "Type A";
    }
}

class ConcreteProductB extends Product {
    public function getType() {
        return "Type B";
    }
}

class ProductFactory {
    public static function create($type) {
        switch ($type) {
            case 'A':
                return new ConcreteProductA();
            case 'B':
                return new ConcreteProductB();
            default:
                throw new Exception("Invalid product type");
        }
    }
}

$product = ProductFactory::create('A');
echo $product->getType(); // Output: Type A

The Factory Method pattern is ideal for scenarios where the class to instantiate should be determined at runtime.

3. Abstract Factory Pattern

The Abstract Factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes. It’s useful when a system needs to be independent of the way its objects are created, composed, and represented.

interface GUIFactory {
    public function createButton();
    public function createCheckbox();
}

class WinFactory implements GUIFactory {
    public function createButton() {
        return new WinButton();
    }
    public function createCheckbox() {
        return new WinCheckbox();
    }
}

class MacFactory implements GUIFactory {
    public function createButton() {
        return new MacButton();
    }
    public function createCheckbox() {
        return new MacCheckbox();
    }
}

class Application {
    private $factory;

    public function __construct(GUIFactory $factory) {
        $this->factory = $factory;
    }

    public function createUI() {
        $button = $this->factory->createButton();
        $checkbox = $this->factory->createCheckbox();
        // Use button and checkbox
    }
}

$factory = new WinFactory();
$app = new Application($factory);
$app->createUI();

The Abstract Factory pattern is particularly useful in situations where a system must support multiple families of products.

Structural Design Patterns

Structural design patterns deal with the composition of classes or objects. These patterns help ensure that if one part of a system changes, the entire structure of the system does not need to change.

1. Adapter Pattern

The Adapter pattern allows objects with incompatible interfaces to work together. It works as a bridge between two incompatible interfaces.

class OldSystem {
    public function oldRequest() {
        return "Old system request";
    }
}

class NewSystem {
    public function newRequest() {
        return "New system request";
    }
}

class Adapter extends NewSystem {
    private $oldSystem;

    public function __construct(OldSystem $oldSystem) {
        $this->oldSystem = $oldSystem;
    }

    public function newRequest() {
        return $this->oldSystem->oldRequest();
    }
}

$oldSystem = new OldSystem();
$adapter = new Adapter($oldSystem);
echo $adapter->newRequest(); // Output: Old system request

The Adapter pattern is useful for integrating legacy code into modern applications without modifying the existing code.

2. Bridge Pattern

The Bridge pattern decouples an abstraction from its implementation so that the two can vary independently. It is useful when both the abstraction and its implementation need to be extended independently.

interface Renderer {
    public function render($text);
}

class HtmlRenderer implements Renderer {
    public function render($text) {
        return "<p>$text</p>";
    }
}

class PlainTextRenderer implements Renderer {
    public function render($text) {
        return $text;
    }
}

abstract class Page {
    protected $renderer;

    public function __construct(Renderer $renderer) {
        $this->renderer = $renderer;
    }

    abstract public function view();
}

class SimplePage extends Page {
    public function view() {
        return $this->renderer->render("Simple Page Content");
    }
}

$htmlRenderer = new HtmlRenderer();
$page = new SimplePage($htmlRenderer);
echo $page->view(); // Output: <p>Simple Page Content</p>

The Bridge pattern is ideal for situations where you want to decouple the abstraction from its implementation, allowing them to be extended independently.

3. Composite Pattern

The Composite pattern allows you to compose objects into tree structures to represent part-whole hierarchies. It lets clients treat individual objects and compositions of objects uniformly.

interface Component {
    public function operation();
}

class Leaf implements Component {
    public function operation() {
        return "Leaf";
    }
}

class Composite implements Component {
    private $children = [];

    public function add(Component $component) {
        $this->children[] = $component;
    }

    public function operation() {
        $result = "Composite(";
        foreach ($this->children as $child) {
            $result .= $child->operation();
        }
        return $result . ")";
    }
}

$leaf1 = new Leaf();
$leaf2 = new Leaf();
$composite = new Composite();
$composite

The Composite pattern is useful for creating hierarchical structures where individual objects and composites of objects are treated uniformly.

Behavioral Design Patterns

Behavioral design patterns focus on how objects interact and communicate with each other. These patterns help to define clear protocols for object interaction and promote flexibility in carrying out communication.

1. Strategy Pattern

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from the clients that use it.

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

class CreditCardPayment implements PaymentStrategy {
    public function pay($amount) {
        echo "Paid $amount using Credit Card";
    }
}

class PayPalPayment implements PaymentStrategy {
    public function pay($amount) {
        echo "Paid $amount using PayPal";
    }
}

class Order {
    private $paymentMethod;

    public function setPaymentMethod(PaymentStrategy $paymentMethod) {
        $this->paymentMethod = $paymentMethod;
    }

    public function checkout($amount) {
        $this->paymentMethod->pay($amount);
    }
}

$order = new Order();
$order->setPaymentMethod(new PayPalPayment());
$order->checkout(100); // Output: Paid 100 using PayPal

The Strategy pattern is beneficial when you have multiple algorithms for a specific task, and you want to switch between them dynamically.

2. Observer Pattern

The Observer pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

class Subject {
    private $observers = [];
    private $state;

    public function attach(Observer $observer) {
        $this->observers[] = $observer;
    }

    public function setState($state) {
        $this->state = $state;
        $this->notifyAllObservers();
    }

    public function getState() {
        return $this->state;
    }

    public function notifyAllObservers() {
        foreach ($this->observers as $observer) {
            $observer->update();
        }
    }
}

interface Observer {
    public function update();
}

class ConcreteObserver implements Observer {
    private $subject;

    public function __construct(Subject $subject) {
        $this->subject = $subject;
        $this->subject->attach($this);
    }

    public function update() {
        echo "Observer State: " . $this->subject->getState();
    }
}

$subject = new Subject();
$observer = new ConcreteObserver($subject);

$subject->setState(1); // Output: Observer State: 1

The Observer pattern is useful for implementing distributed event handling systems.

3. Command Pattern

The Command pattern turns a request into a stand-alone object that contains all information about the request. This transformation lets you parameterize methods with requests, delay or queue a request’s execution, and support undoable operations.

interface Command {
    public function execute();
}

class Light {
    public function on() {
        echo "Light is ON";
    }

    public function off() {
        echo "Light is OFF";
    }
}

class LightOnCommand implements Command {
    private $light;

    public function __construct(Light $light) {
        $this->light = $light;
    }

    public function execute() {
        $this->light->on();
    }
}

class LightOffCommand implements Command {
    private $light;

    public function __construct(Light $light) {
        $this->light = $light;
    }

    public function execute() {
        $this->light->off();
    }
}

class RemoteControl {
    private $command;

    public function setCommand(Command $command) {
        $this->command = $command;
    }

    public function pressButton() {
        $this->command->execute();
    }
}

$light = new Light();
$lightOn = new LightOnCommand($light);
$lightOff = new LightOffCommand($light);

$remote = new RemoteControl();
$remote->setCommand($lightOn);
$remote->pressButton(); // Output: Light is ON

$remote->setCommand($lightOff);
$remote->pressButton(); // Output: Light is OFF