PHP Inheritance

Class Inheritance & Method Overriding (extends, parent::)

Inheritance allows a child subclass to inherit all public and protected properties and methods from a parent superclass using the extends keyword. Child classes can extend functionality, add new methods, or override parent behaviors using parent::.

Inheritance Structure

flowchart TD
    A["Parent Class (Vehicle)"] --> B["Child Subclass 1 (Car)"]
    A --> C["Child Subclass 2 (ElectricCar)"]
    A --> A1["#speed, +accelerate()"]
    B --> B1["+fuelType, +refuel()"]
    C --> C1["+batteryCapacity, +chargeBattery()"]

Keyword Rules

• extends: Establishes inheritance link.
• parent::__construct(): Calls parent constructor from child class.
• final: Prevents a class from being extended or a method from being overridden.

Practical Code Example

<?php
declare(strict_types=1);

class Vehicle
{
    public function __construct(
        protected string $brand,
        protected int $speed = 0
    ) {}

    public function accelerate(int $increment): void
    {
        $this->speed += $increment;
        echo "{$this->brand} accelerated to {$this->speed} km/h.
";
    }

    public function getInfo(): string
    {
        return "Brand: {$this->brand} | Current Speed: {$this->speed} km/h";
    }
}

class ElectricCar extends Vehicle
{
    public function __construct(
        string $brand,
        public int $batteryCapacityKWh
    ) {
        // Call parent constructor
        parent::__construct($brand);
    }

    // Overriding parent getInfo() method
    public function getInfo(): string
    {
        return parent::getInfo() . " | Battery: {$this->batteryCapacityKWh} kWh";
    }
}

$ev = new ElectricCar("Tesla", 85);
$ev->accelerate(50);
echo $ev->getInfo() . "
";

Best Practices

• Call parent::__construct() in Child Constructors: Ensures parent initialization logic runs cleanly.
• Use final to Prevent Unwanted Inheritance: Mark classes or critical security methods as final class UserSecurity to prevent overriding.
• Adhere to Liskov Substitution Principle (LSP): Child classes must remain compatible with parent method signatures and behaviors.

Self-Check Challenge

Write a class Manager extends Employee that calls parent::__construct($name, $salary) inside its constructor!