php用mysqli模块连数据库Fatal error: Call to a member function query() 。解决思路

<?php        class Database    {        var $mysqli;        var $showError;                public function _construct($configFile="config.inc.php")        {            require_once($configFile);            $this->mysqli=new mysqli($dbhost,$dbuser,$dbpwd,$dbname);            if(mysqli_connect_errno())            {                echo "物品数据库连接失败！";            }        }        public function close()        {            if($this->mysqli)                $this->mysqli->close();            $this->mysqli=FALSE;        }        public function _destruct()        {            $this->close();        }    }    class UserDB extends Database    {        public function queryUserByName($name)        {            $query="SELECT * FROM Users WHERE name='".$name."'";            if ($result=$this->mysqli->query($query))  //此处提示出错！！                return true;            else                 return false;        }

<?php    function __autoload($className)    {        include_once "../model/".$className.".class.php";    }    $udb=new UserDB();   //明明初始化了，可是到类里面就出错？    $idb=new ItemsDB();       echo "jinlaile";//    if(isset($_COOKIE("userInfo")))//    {//        $userInfo=unserialize($_COOKIE("userInfo"));//    }    switch($_GET["action"])    {        case "register":                        $lvalue=$_POST;            $rtmp=$udb->queryUserByName($lvalue["name"]);            echo "alert";            if($rtmp)  //这个名字没有注册            {                $rusers=new User($lvalue);                if($udb->addUser($rusers))                {                    setcookie("start",0);                    setcookie("userInfo",serialize($rusers));                    setcookie("username",$rusers->name);                    header("Location:../view/index.php");                }                else                     header("Location:../view/errors.php?value=tianjia");            }            break;       }