为甚麽这个变量为null

class my_connect{       private $conn;       private  function get_conn() {            $conn=new mysqli('localhost',"root","","test",3306);            $conn->set_charset("utf8");            return $conn;        }        private  function __destruct() {            if(isset($this->conn)){                $this->conn->close();            }        }        static function get_instance() {            $new_conn=new my_connect() ;            $conn=$new_conn->get_conn() ;            return $conn;        }    } 当我执行 $conn=my_connect::get_instance(); 到析构函数的时候 $conn变量为null也就是没有执行  $this->conn->close();

if(isset($this->conn)){

if($this->conn){

private  function get_conn

public  function get_conn