PHP Abstract Classes

Key Concepts

// This will cause error
$obj = new Animal();

abstract public function 
  makeSound();

public function sleep() {
  echo "Sleeping...";
}

class Dog extends Animal {
  // Must implement
}

🔹 Basic Abstract Class

Abstract classes serve as blueprints for child classes. They can contain both abstract methods (without body) and regular methods (with body). Child classes must implement all abstract methods to work properly.

<?php
// Define abstract class
abstract class Animal {
    protected $name;
    
    // Regular method
    public function setName($name) {
        $this->name = $name;
    }
    
    // Abstract method - no body
    abstract public function makeSound();
}

// Child class must implement abstract methods
class Dog extends Animal {
    public function makeSound() {
        return "Woof! Woof!";
    }
}

class Cat extends Animal {
    public function makeSound() {
        return "Meow! Meow!";
    }
}

// Create objects
$dog = new Dog();
$dog->setName("Buddy");
echo $dog->makeSound(); // Woof! Woof!

$cat = new Cat();
$cat->setName("Whiskers");
echo $cat->makeSound(); // Meow! Meow!
?>

🔹 Abstract Class with Properties

Abstract classes can have properties and constructors just like regular classes. This allows you to share common data and initialization logic across all child classes while enforcing specific method implementations.

<?php
abstract class Shape {
    protected $color;
    
    public function __construct($color) {
        $this->color = $color;
    }
    
    // Abstract methods
    abstract public function calculateArea();
    abstract public function getName();
    
    // Regular method
    public function getColor() {
        return $this->color;
    }
}

class Circle extends Shape {
    private $radius;
    
    public function __construct($color, $radius) {
        parent::__construct($color);
        $this->radius = $radius;
    }
    
    public function calculateArea() {
        return 3.14 * $this->radius * $this->radius;
    }
    
    public function getName() {
        return "Circle";
    }
}

class Square extends Shape {
    private $side;
    
    public function __construct($color, $side) {
        parent::__construct($color);
        $this->side = $side;
    }
    
    public function calculateArea() {
        return $this->side * $this->side;
    }
    
    public function getName() {
        return "Square";
    }
}

// Create shapes
$circle = new Circle("Red", 5);
echo $circle->getName() . " is " . $circle->getColor();
echo " with area: " . $circle->calculateArea();

$square = new Square("Blue", 4);
echo $square->getName() . " is " . $square->getColor();
echo " with area: " . $square->calculateArea();
?>

🔹 Real-World Example

Abstract classes are perfect for creating payment systems where different payment methods share common features but have unique processing logic. This ensures all payment types follow the same structure while implementing their specific requirements.

<?php
abstract class Payment {
    protected $amount;
    
    public function __construct($amount) {
        $this->amount = $amount;
    }
    
    // Abstract method
    abstract public function processPayment();
    
    // Regular method
    public function getAmount() {
        return $this->amount;
    }
}

class CreditCard extends Payment {
    private $cardNumber;
    
    public function __construct($amount, $cardNumber) {
        parent::__construct($amount);
        $this->cardNumber = $cardNumber;
    }
    
    public function processPayment() {
        return "Processing $" . $this->amount . 
               " via Credit Card ending in " . 
               substr($this->cardNumber, -4);
    }
}

class PayPal extends Payment {
    private $email;
    
    public function __construct($amount, $email) {
        parent::__construct($amount);
        $this->email = $email;
    }
    
    public function processPayment() {
        return "Processing $" . $this->amount . 
               " via PayPal account: " . $this->email;
    }
}

// Process payments
$cc = new CreditCard(100, "1234567890123456");
echo $cc->processPayment();

$pp = new PayPal(50, "[email protected]");
echo $pp->processPayment();
?>