PHPオブジェクトのコピー

<?php
    class SubObject
    {
        static $instances = 0;
        public $instance;
        public function __construct() {
            $this->instance = ++self::$instances;
        }
        public function __clone() {
            $this->instance = ++self::$instances;
        }
    }
    class MyCloneable
    {
        public $object1;
        public $object2;
        function __clone()
        {
            // 强制复制一份this->object， 否则仍然指向同一个对象
            $this->object1 = clone $this->object1;
        }
    }
    $obj = new MyCloneable();
    $obj->object1 = new SubObject();
    $obj->object2 = new SubObject();
    $obj2 = clone $obj;
    print("Original Object:\n");
    print_r($obj);
    print("Cloned Object:\n");
    print_r($obj2);
?>

Original Object:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 1
        )
    [object2] => SubObject Object
        (
            [instance] => 2
        )
)
Cloned Object:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 3
        )
    [object2] => SubObject Object
        (
            [instance] => 2
        )
)