PHP — Inheritance

Basic inheritance

<?php
class Animal {
    public string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }

    public function speak(): string {
        return "The animal speaks";
    }
}

class Dog extends Animal {
    public function speak(): string {
        return "{$this->name} barks";
    }
}

$dog = new Dog("Rex");
echo $dog->speak(); // Rex barks
?>

Protected and private

<?php
class Parent {
    protected int $x = 10;
    private int $y = 20;

    public function getX(): int { return $this->x; }
}

class Child extends Parent {
    public function show(): void {
        echo $this->x;  // OK: protected
        // echo $this->y; // Error: private
    }
}
?>

Constructor chaining

<?php
class Person {
    public string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }
}

class Employee extends Person {
    public string $company;

    public function __construct(string $name, string $company) {
        parent::__construct($name);
        $this->company = $company;
    }
}

$emp = new Employee("Alice", "TechCorp");
echo "{$emp->name} works at {$emp->company}";
?>

Final classes and methods

<?php
final class Immutable {
    public function __construct(public string $value) {}
}

// class Child extends Immutable {} // Error: cannot extend final

class Base {
    final public function show(): void {
        echo "Cannot override";
    }
}

class Child extends Base {
    // public function show(): void {} // Error: cannot override final
}
?>

Abstract classes

<?php
abstract class Shape {
    abstract public function area(): float;

    public function describe(): string {
        return "Area: " . number_format($this->area(), 2);
    }
}

class Circle extends Shape {
    public function __construct(private float $radius) {}

    public function area(): float {
        return M_PI * $this->radius ** 2;
    }
}

$circle = new Circle(5);
echo $circle->describe(); // Area: 78.54
?>

Mini Practice

Write PHP code that:

1. Creates a parent and child class
2. Uses protected properties
3. Chains constructors with parent::__construct()
4. Demonstrates final class

Up Next

In the next lesson, you'll learn about Interfaces — defining contracts in PHP.