lol2.php

<?php

// PRIVATE

class A {
    private $_a = 'foo';

    public function get_a() {
        return $this->_a;
    }
}

class B extends A {
    public function set_a($x) {
        $this->_a = $x;
    }

    public function set_other_a($a, $x) {
        $a->_a = $x;
    }
}

$a = new A();

$b = new B();

// doesn't work because A::_a is private and thus invisible to B objects
$b->set_a('bar');

// also doesn't work for the same reason
$b->set_other_a($a, 'bar');

// this is all fine and expected

// PROTECTED

class C {
    protected $_a = 'foo';

    public function get_a() {
        return $this->_a;
    }
}

class D extends C {
    public function set_a($x) {
        $this->_a = $x;
    }

    public function set_other_a($a, $x) {
        $a->_a = $x;
    }
}

$c = new C();

$d = new D();

// works, is fine
$d->set_a('bar');

// works, but shouldn't because it lets any object that extends your class
// fuck with your objects
$d->set_other_a($c, 'bar');