警告：mysqli_close（）期望参数1为mysqli，null为1

Question

首先抱歉我的英语:)你告诉我为什么当我调用魔法__destructor（）时我有这个警告，我不认为这是属性连接的范围，帮助我...谢谢大家！

<?php

class Connect{

 protected $user;
 protected $pass;
 protected $host;
 protected $db;
 protected $query;
 protected $result;
 private $connex;


 function __construct($host,$user,$pass,$database){

  $this->host=$host;
  $this->user=$user;
  $this->pass=$pass;
  $this->db=$database;



  // connesione al database

  $this->connex = new mysqli("$this->host", "$this->user", "$this->pass", "$this->db");
  if ($this->connex->connect_errno) {
   echo "Failed to connect to MySQL: (" . $this->connect_errno . ") " . $this->connect_error;
  }
  // echo 'connessione stabilita' . "\n";

 }  


function __destruct(){
 mysqli_close($this->connex);
 $this->connex = null; 
}


}



$ist = new Connect("localhost","root","","mio");
$ist->__destruct();