为什么可以从对象范围外调用私有方法？

Question

I am curious as to why this is allowed to work, whereby you can call and successfully execute a private method on an object from outside of the object scope providing you are making the call from a class of the same type.

The private method call from a public scope to me seems not to satisfy the criteria of a private method, so why is this allowed in both PHP and Java?

<?php

class A
{

    public function publicMethod ()
    {
        $obj = new static;
        $obj->privateMethod ();
    }

    private function privateMethod ()
    {
        echo 'why does this execute?';
    }

}

$obj = new A;
$obj->publicMethod ();

Answer 1

Objects of the same type will have access to each others private and protected members even though they are not the same instances. This is because the implementation specific details are already known when inside those objects.

-- Visiblity, PHP Manual