<?php
class A { }

$a = new A;
$b = $a;  

$a->age = 10;
$b->age = 20;

echo $a->age; // ??????????

<?php

class A { 
  public int $age;
}

class B {
  public A $a;
}

$a = new A;
$b = new B;


$a->age = 0;
$b->a = $a;
$b->a->age = 5;
$a->age = 10;


echo $b->a->age; // ??????????