php调用数据库操作类的问题！


class db_class{
    public $db_url; //连接地址
    public $db_username; //连接名
    public $db_userpassword; //连接密码
    public $db_name; //数据库名
    public $db_tablename; //表名
    public $db_conn; //数据库连接
    public $db_order;
    public $db_limit;
    public function db_getconn(){ //连接数据库
        $this->db_conn= mysqli_connect($this->db_url,$this->db_username,$this->db_userpassword,$this->db_name);
        if (!$this->db_conn)
        {
      echo "连接 MySQL 失败: " . mysqli_connect_error();
    }
    }
    public function __construct($db_url,$db_username,$db_userpassword,$db_name){ //构造方法赋值
    $this->db_url=$db_url;
        $this->db_username=$db_username;
        $this->db_userpassword=$db_userpassword;
        $this->db_name=$db_name;
        $this->db_order="";
        $this->db_limit="";
        $this->db_getconn();
    mysqli_query($this->db_conn,'set names utf8');
    }
    public function db_settablename($db_tablename){ //设置表名
        $this->db_tablename=$db_tablename;
    }
    public function db_setorder($str){ //排序操作
        $this->db_order="order by $str";
    }
    public function db_setlimit($start,$end){ //分页操作
     $this->db_limit="limit $start,$end";
    }
    public function db_select($typearr="",$where=""){ //查询操作
        if(empty($typearr)){
            $typearr="*";
        }
        else{
            $typearr=implode(",",$typearr);
        }
        if(empty($where)){
            $where="";
        }else{
            $where="where ".$where;
        }
        $arr=array();
        $sql="select $typearr from $this->db_tablename $where $this->db_order $this->db_limit ";
        $result = mysqli_query($this->db_conn,$sql);
        while($row = $result->fetch_array()) {
            $arr[]=$row;
        }
        return $arr;

    }
    public function db_update($typearr,$valuearr,$where=""){ //更新操作
    $sql="";
        if(empty($where)){
            $where="";
        }else{
            $where=" where ".$where;
        }
        $sql.="update $this->db_tablename set ";
         foreach ($typearr as $key=>$value){
             if(count($typearr)-1==$key){
                 $sql.=$value."='".$valuearr[$key]."'";
             }else{
                 $sql.=$value."='".$valuearr[$key]."'".",";
             }
         }
        $sql.=$where;
    mysqli_query($this->db_conn,$sql);
    }
    public function db_delete($typestr,$valuestr){ //删除操作
    $sql="delete from $this->db_tablename WHERE $typestr=$valuestr";
    mysqli_query($this->db_conn,$sql);
    }
    public function db_insert($typearr,$valuearr){ //插入操作
        $sql="insert into $this->db_tablename(".implode(",", $typearr).") values(".implode(",", $valuearr).")";
    mysqli_query($this->db_conn,$sql);
    }
    public function __destruct(){ //析构方法关闭连接
        mysqli_close($this->db_conn);
    }
}
$db=new db_class('localhost','root','123456','test','news');$db->db_select('',''); //报错 在调用时，请大家帮我看下问题出在哪里了？