PHP 实现的双向链表，数据存放原理？ [ 未指定版本 ]

class link
{
    public $preLink = NULL; //前指针
    public $number;         //排名
    public $name;               //名字
    public $nextLink = NULL;    //后指针

    //构造函数，初始化值
    public function __construct($number= '', $name = '')
    {
       $this->number = $number;
       $this->name = $name;
   }
   //增加数据
   static public function addDom($head, $data){
       $obj = $head;
       $isExist = FALSE;
       if(NULL == $obj->nextLink)
       {
           $obj->nextLink = $data;
           $data->preLink = $head;
       }
        //找到添加的位置
        while(NULL != $obj->nextLink){
            if($obj->nextLink->number >$data->number){
                break;
            }else if($obj->nextLink->number == $data->number){
                $isExist = TRUE;
                echo "<br>不能添加相同的编号";
            }
            $obj = $obj->nextLink;
        }
        if(!$isExist){
            if(NULL !=$obj->nextLink){
                  $data->nextLink =$obj->nextLink;
            }
            $data->preLink = $obj;
            if(NULL !=$obj->nextLink){
                $data->nextLink->preLink = $data;
            }
            $obj->nextLink = $data;
        }
    }

    //输出链表数据
    static public function showDom($head) {
        $obj = $head;
        while(NULL != $obj->nextLink){
        echo "<br>编号：".$obj->nextLink->number."名字：".$obj->nextLink->name;
           $obj = $obj->nextLink;
        }
    }

    static public function delDom($head, $num){
        $obj = $head;
        $isFind = FALSE;
        while(NULL != $obj){
            if($obj->number == $num){
               $isFind = TRUE;
                break;
            }
            $obj = $obj->nextLink;
        }
        if($isFind){
            if(NULL !=$obj->nextLink){
                $obj->preLink->nextLink = $obj->nextLink;  
                echo '<br/>删除的号码是'.$obj->number;

            }else{
                echo "<br>没有找到目标";
            }            
        }
    }


    static public function updateDom($head, $num, $name){
        $obj = $head;
        while(NULL != $obj){
            if($obj->number == $num){
                $obj->name = $name;
                echo "<br/>改变号码{$obj->number}的结果：".$obj->name;
                break;
            }
            $obj = $obj->nextLink;
        }
    }
}



$head = new link();
$one = new link(1,'oooooooo');
$two = new link(2,'wwwwwwww');
$three = new link(3,'eeeeeeee');
link::addDom($head,$one);
link::addDom($head,$two);
link::addDom($head,$three);

link::showDom($head);
link::delDom($head, 2);

link::showDom($head);
link::updateDom($head,3, 'kkkkkk');
link::showDom($head);

数据是存放在哪里的？是存放在堆栈里吗？
link::addDom($head,$one);
link::addDom($head,$two); 这样存不会覆盖上一个吗？
是什么样的存储原理？