public-parameters-oop

<?php

ini_set('display_errors', 1); 
ini_set('display_startup_errors', 1); 
error_reporting(E_ALL);

class Car {

	public $color = 'White';

	public function setColor($color)
	{
		$this->color = $color;
	}

	public function getColor()
	{
		return $this->color;
	}

}

$car = new Car();
echo $car->color; 			// return White

$car->setColor('Yellow');
echo $car->getColor();			// return Yellow

////////////////////////////////////////////////////////////////

class Beetle extends Car {



}

$beetle = new Beetle();
echo $beetle->color;			// return White. The $color is instantiated
					// in class Car() and not $color in Beetle()

$beetle->setColor('Green');
echo $beetle->color;			// return Green
echo $beetle->getColor();		// return Green

////////////////////////////////////////////////////////////////

class BeetleGT extends Beetle {



}

$beetleGT = new Beetle();
echo $beetleGT->color;			// return White. The $color is instantiated
					// in class Car() and not $color in Beetle() or BeetleGT()

$beetleGT->setColor('Orange');
echo $beetleGT->color;			// return Orange
echo $beetleGT->getColor();		// return Orange

////////////////////////////////////////////////////////////////

class Ferrari extends Car {

	public $color; // sets value as null

}

$ferrari = new Ferrari();
echo $ferrari->color;			// return null. The $color is instantiated in Ferrari()

$ferrari->setColor('Red');
echo $ferrari->color;			// return Red
echo $ferrari->getColor();		// return Red

////////////////////////////////////////////////////////////////

class FerrariF40 extends Ferrari {



}

$ferrariF40 = new FerrariF40();
echo $ferrariF40->color;		// return null. The $color is instantiated in Ferrari()

$ferrariF40->setColor('FerrariRed');
echo $ferrariF40->color;		// return FerrariRed
echo $ferrariF40->getColor();		// return FerrariRed

Article explaining it: https://medium.com/@cesarkohl/public-parameters-in-oop-11a6d5b355a6