Inheritance and Interfaces — PHP Tutorial

Inheritance lets one class build on another; interfaces let unrelated classes promise to support the same set of methods.

Extending a class

<?php
class Animal {
    public function __construct(protected string $name) {}

    public function makeSound(): string {
        return "...";
    }
}

class Dog extends Animal {
    public function makeSound(): string {
        return "{$this->name} says Woof!";
    }
}

$dog = new Dog("Rex");
echo $dog->makeSound(); // "Rex says Woof!"

extends makes Dog a subclass of Animal, inheriting its constructor and properties automatically. makeSound() is overridden — Dog's version replaces Animal's for any Dog instance, a mechanism called polymorphism: code that just calls $animal->makeSound() gets the right behavior regardless of which subclass $animal actually is.

Calling the parent's implementation

<?php
class Employee {
    public function __construct(protected string $name, protected float $baseSalary) {}

    public function getSalary(): float {
        return $this->baseSalary;
    }
}

class Manager extends Employee {
    public function __construct(string $name, float $baseSalary, private float $bonus) {
        parent::__construct($name, $baseSalary);
    }

    public function getSalary(): float {
        return parent::getSalary() + $this->bonus;
    }
}

parent:: calls the parent class's version of a method (or its constructor) explicitly — useful when a subclass wants to extend behavior rather than fully replace it, as Manager::getSalary() does here by adding a bonus on top of the inherited calculation.

Abstract classes

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

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

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

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

An abstract class can't be instantiated directly (new Shape() is an error) and can declare abstract methods with no body — every concrete subclass is required to implement them. describe() is a normal, inherited method that works for any subclass, since it only depends on area() existing, not on how each shape calculates it.

Interfaces: a contract without implementation

<?php
interface Payable {
    public function calculatePayment(): float;
}

class Freelancer implements Payable {
    public function __construct(private float $hourlyRate, private int $hours) {}

    public function calculatePayment(): float {
        return $this->hourlyRate * $this->hours;
    }
}

function processPayment(Payable $entity): void {
    echo "Paying: " . $entity->calculatePayment();
}

An interface declares method signatures with no implementation at all — a class implements it as a promise that it provides those methods. Unlike extends (single inheritance only — a class can extend just one parent), a class can implements several interfaces at once, which is how PHP achieves something like multiple inheritance for behavior contracts without the complexity of true multiple class inheritance.

Extending a class

<?php
class Animal {
    public function __construct(protected string $name) {}

    public function makeSound(): string {
        return "...";
    }
}

class Dog extends Animal {
    public function makeSound(): string {
        return "{$this->name} says Woof!";
    }
}

$dog = new Dog("Rex");
echo $dog->makeSound(); // "Rex says Woof!"

Calling the parent's implementation

<?php
class Employee {
    public function __construct(protected string $name, protected float $baseSalary) {}

    public function getSalary(): float {
        return $this->baseSalary;
    }
}

class Manager extends Employee {
    public function __construct(string $name, float $baseSalary, private float $bonus) {
        parent::__construct($name, $baseSalary);
    }

    public function getSalary(): float {
        return parent::getSalary() + $this->bonus;
    }
}

Abstract classes

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

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

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

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

Interfaces: a contract without implementation

<?php
interface Payable {
    public function calculatePayment(): float;
}

class Freelancer implements Payable {
    public function __construct(private float $hourlyRate, private int $hours) {}

    public function calculatePayment(): float {
        return $this->hourlyRate * $this->hours;
    }
}

function processPayment(Payable $entity): void {
    echo "Paying: " . $entity->calculatePayment();
}