Inheritance Principle

Inheritance is a way of extending class. The new class (child class) builds logic on the top of the parent class (super class).

Here's an example:

<?php
// Base class:
class Vehicle
{
    protected $make;
    protected $model;

public function representation()
    {
        echo $this->make . " " . $this->model . "\n";
    }

public function getInsuranceCost($months)
    {
        return 0;
    }

public function getMake()
    {
        return $this->make;
    }

public function getModel()
    {
        return $this->model;
    }
}

// Car inherits Vehicle:
class Car extends Vehicle
{
    // Adds class member:
    private $doorsCount;

// Overwriting method:
    public function getInsuranceCost($months)
    {
        return $months * 25;
    }

// Adding constructor:
    public function __construct($makeValue, $modelValue,
    $doorsCountValue)
    {
        $this->make = $makeValue;
        $this->model = $modelValue;
        $this->doorsCount = $doorsCountValue;
    }

// Method extension:
    public function representation()
    {
        parent::representation();
        echo "Doors count: " . $this->doorsCount . "\n";
    }

// Adding new method:
    public function getDoorsCount()
    {
        return $this->doorsCount;
    }
}

$car = new Car("Toyota", "Corolla", 4);

// Method that was extended:
$car->representation();
echo "------\n";

// Method that was added:
echo $car->getDoorsCount() . "\n";
echo"------\n";

// Methods that weren't changed:
echo "Make: " . $car->getMake() . "\n";
echo "Model: " . $car->getModel() . "\n";
echo"------\n";

// Method that was overwritten:
echo "Insurance cost: " .
            $car->getInsuranceCost(12) . " $";

Output:

Basically the child class can add, extend, rewrite or use the inherited logic from the parent.

For this example:

Added logic:
- member - doorsCount (line 33)
- parametrized constructor (line 42)
- getter for doorsCount (line 58)
Extended logic:
- representation method (line 51)
Entirely rewritten logic:
- getInsuranceCost (line 36)
Not modified logic:
- getMake (line 18)
- getModel (line 23)