为什么这个会连接不上呢？

代码如下

<?php
class ConnDB{
    private $localhost;
    private $username;
    private $pwd;
    private $dbname;
    private $coon;
    private $code;
    function _construct($localhost,$username,$pwd,$dbname,$coon, $code){
        $this->localhost=$localhost;
        $this->username=$username;
        $this->pwd=$pwd;
        $this->dbname=$dbname;
        $this->code=$code;
    }
    function connect(){
        $this->conn=mysqli_connect($this->localhost,$this->username, $this->pwd,$this->dbname)or die("Connect MySQL False");
        mysqli_query($this->conn,"set names $this->code");
        return $this->conn;
        }

使用php -m命令，查看mysqli拓展是否有开启