Private property in parent class can be accessed and re-assigned through the instance of child class in PHP but protected one can't

Question

I am very confused why $produk1->harga = 500; can still make a change (or re-assign the value 500) to private $harga property despite private $harga in class Produk has PRIVATE visibility ? $product1 is an instance of class Komik.

$produk1 = new Komik("Naruto", "Masashi Kishimoto", "Shonen Jump", 30000, 100);

And by echo $produk1->harga; it prints out 500 and not an ERROR. But when I change the visibility to protected $harga it prints out ERROR. How come? I don't understand. Thank you for the answers.

    <?php
        
        class Produk {
            public $judul = "judul",
                    $penulis = "penulis",
                    $penerbit = "penerbit";
            private $harga;
        
        
            public function __construct($judul, $penulis, $penerbit, $harga) {
                $this->judul = $judul;
                $this->penulis = $penulis;
                $this->penerbit = $penerbit;
                $this->harga = $harga;
            }
        
            public function detailInfo() {
                $str = "{$this->judul} | {$this->penulis}, {$this->penerbit} (Rp.{$this->harga})";
        
                return $str;
            }
        
        }
        
        class Komik extends Produk {
            public $jumlahHalaman = 0;
        
            public function __construct($judul, $penulis, $penerbit, $harga, $jumlahHalaman) {
        
                parent::__construct($judul, $penulis, $penerbit, $harga);
                $this->jumlahHalaman = $jumlahHalaman;
            }
        
            public function detailInfo() {
                $str = "Komik : " . parent::detailInfo() . " - {$this->jumlahHalaman} halaman.";
        
                return $str;
            }
        }
        
        
        class Game extends Produk {
        
            public $jumlahDurasi = 0;
        
            public function __construct($judul, $penulis, $penerbit, $harga, $jumlahDurasi) {
                
                parent::__construct($judul, $penulis, $penerbit, $harga);
                $this->jumlahDurasi = $jumlahDurasi;
            }
        
            public function detailInfo() {
                $str = "Game : " . parent::detailInfo() . " ~ {$this->jumlahDurasi} jam.";
        
                return $str;
            }
        }
        
        
        $produk1 = new Komik("Naruto", "Masashi Kishimoto", "Shonen Jump", 30000, 100);
        $produk2 = new Game("Uncharted", "Neil Druckmann", "Sony Computer", 250000, 5);
        
        
        echo $produk1->detailInfo();
        echo "<br>";
        echo $produk2->detailInfo();
        
        echo "<hr>";
        $produk1->harga = 500;
        echo $produk1->harga;

OUTPUT =

Komik : Naruto | Masashi Kishimoto, Shonen Jump (Rp.30000) - 100 halaman. Game : Uncharted | Neil Druckmann, Sony Computer (Rp.250000) ~ 5 jam.

---

500

IF $harga property visibility is protected

    <?php
        
        class Produk {
            public $judul = "judul",
                    $penulis = "penulis",
                    $penerbit = "penerbit";
            protected $harga;
.
.
.

OUTPUT =

Fatal error: Uncaught Error: Cannot access protected property Komik::$harga in ...

Error: Cannot access protected property Komik::$harga in ...

Answer 1

It's because private properties are not inherited, and when you are trying to access your private property from child class - PHP can't find it and dynamically creates new public one. See example below. Protected properties are inherited and that's why you can't access them from other parts of code.

<?php

class A {
    private $property = 100;
    
    public function print() {
        echo $this->property;
    }
}

class B extends A {}

$b = new B();
$b->property = 500;
$b->print(); // 100

var_dump($b);

/* 
object(B)#1 (2) {
  ["property":"A":private] => int(100)
  ["property"]             => int(500)
} 
*/

Answer 2

Protected variable can be accessed inside child class. However, you are not accessing it inside child class rather your accessing it outside the child class.

You can access the protected member by making a getter() function like

public function getHarga()
{
    return $this->$harga;
}

You can see the doc for visibility.