PHP:引用对象的链表实现问题

我正在创建一个链表类，并且对对象的引用有一些混淆。

根据我的理解，默认情况下，对象是通过引用复制的$Obj1=$Obj2$Obj1是$Obj2的别名。

有人能指出链接列表实现中哪一个是正确的吗。

$firstNode->next = $this->first;---> seems to be correct
             or 
$firstNode->next =& $this->first;
$this->first = $firstNode;-----> seems to be correct as $firstNode is an object
             or 
$this->first = & $firstNode;

代码：

class Node {
    public $element;
    public $next;
public function __construct($element){
    $this->element = $element;
    $this->next = NULL;
  }
}
class Linklist {
    private $first;
    private $listSize;
 public function __construct(){      
    $this->first = NULL;
    $this->listSize = 0;
}
public function InsertToFirst($element){
     $firstNode = new Node($element);
     $firstNode->next = $this->first;   // or $firstNode->next =& $this->first;
     $this->first = $firstNode;  // or $this->first = & $firstNode;
 }