private access in PHP / encapsulation hack

encapsulation_hack.php

<?php
declare(strict_types=1);

final class A {
    private $prop;
    public function __construct(DateTime $prop) { $this->prop = $prop; }
    public function getProp() { return $this->prop; }
}

$a = new A(new DateTime());
var_dump($a->getProp());

(function (){
    return $this->prop = "injection success";
})->call($a);

var_dump($a->getProp());

RESULT

object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2018-01-31 01:51:23.457999"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(10) "US/Pacific"
}
string(17) "injection success"