Private access in inheritance

Question

class  Person {
    public  $name;
    private $age; //private access
}

class Employee extends Person{
    public $id;
    public $salary; //class property   
}

$emp = new Employee(); 
$emp->name="ABCD";
$emp->age = 30;
$emp->id=101;
$emp->salary=20000;

echo "<br/> Name : ".$emp->name;
echo "<br/> Age : ".$emp->age;

In this code, the child class variable $emp can access private member of parent class Person directly. Is this not the violation of private access rule?

It gives error when using a parent class variable, but works with child class variable!! Can anyone explain why?

Answer 1

TLDR;

$emp->age = 30 does not call parent private member age, it creates new child object property age dynamically.

Explanation

Looks like a bug, doesn't it? Firstly, let's comment out the parent's private member:

<?php

class Person {
    // private $age;
}

class Employee extends Person {
}

$emp = new Employee();
$emp->age = 10;
echo $emp->age . "\n";
//out: 10

In the line $emp->age = 10 we created new property of $emp object named age and assigned it value 10.

When you define your parent's member as private, children don't see this member at all:

<?php

class  Person {
    private $age;

    function __construct() {
        $this->age = 30;
    }

    public function printAge()
    {
        echo sprintf("Parent::age = %s\n", $this->age);
    }
}

class Employee extends Person {
    private $age;

    function __construct() {
        parent::__construct();
        $this->age = 10;
    }

    public function printAge()
    {
        echo sprintf("Employee::age = %s\n", $this->age);
        parent::printAge();
    }
}

$emp = new Employee();
$emp->printAge();

//out:
//Employee::age = 10
//Parent::age = 30