Careful with immutability

careful_with_immutability.php

<?php
declare(strict_types=1);

class X {
    private $s;
    public function __construct(string $s) { $this->s = $s; }
    public function getS(): string { return $this->s; }
}
class Y {
    private $x;
    public function __construct(X $x) { $this->x = $x; }
    public function getX(): X { return $this->x; }
}

$x = new X('foo');
$y1 = new Y($x);
$y2 = new Y($x);

assert(spl_object_hash($y1) !== spl_object_hash($y2)); // top object differ
assert(spl_object_hash($y1->getX()) === spl_object_hash($y2->getX())); // child object stay the same

$y1X = $y1->getX();
$data = (new ReflectionClass($y1X))->getProperty('s');
$data->setAccessible(true);
$data->setValue($y1X, 'bar');

assert($data->getValue($y1X) === $y1->getX()->getS()); // value is indeed changed
assert($y1->getX()->getS() === $y2->getX()->getS()); // and it also changed in the other