PHP 中的对象传递

<?php
class A
{
    public $age = 0;
    public $username = "";

    public $obj = null;
}

$a = new A();
$b = $a;

$b->age = 1;

var_dump($b);
var_dump($a);

debug_zval_dump($a);
echo "<br/>";
debug_zval_dump($b);