PHP调用自己定义的一个数据库类时出现错误？

本人初学，正在尝试自己做一个论坛，很多地方不懂，这是我定义的数据库类：
<?PHP
class content{
  var $conn;
public $contid;
public $subject;
public $words;
public $username;
public $face;
public $email;
public $homepage;
public $createtime;
public $upperid;

function _construct(){
  $this->mysqli_connect("localhost","root","","book");
mysqli_query($this->conn,"SET NAMES gbk");
}
  function _destruct(){
  mysqli_close($this->conn);
}
function getinfo($id){
$sql="select * from content where contid='".$id;
$result=$this->conn->query($sql);
if($row=$result->fetch_row())
{
$this->contid=$id;
$this->subject=$row[1];
$this->words=$row[2];
$this->username=$row[3];
$this->face=$row[4];
$this->email=$row[5];
$this->homepage=$row[6];
$this->createtime=$row[7];
$this->upperid=(int)$row[8];
}
}
function getrecordcount(){
$sql="select count(*) from content";
$result=$this->conn->query($sql);
if($row=$result->fetch_row())
return (int)$row[0];
else
return 0;
}
function insert(){
$sql="insert into content (subject,words,username,face,email,homepage,createtime,upperid) values ('".$this->subject."','".$this->words."','
".$this->username."','".$this->face."','".$this->email."','".$this->homepage."','".$this->createtime."','".$this->upperid."')";
$this->conn->query($sql);
}
function delete($id){
$sql="delete from content where contid=".$id."or upperid".$id;
$this->conn->query($sql);
}
function load_content_byupperid($uid){
$sql="select * from content where upperid=".$uid."order by createtime desc";
$result=$this->conn->query($sql);
return $result;
}
function load_content_bypage($pageno,$pagesize){
$sql="select * from content order by createtime desc limit".($pageno-1)*$pagesize.",".$pagesize;
$result=$this->conn->query($sql);
return $result;
}
}
?>
然后在另一个网页使用，include调入上面这个网页时，出现了这样一个错误：Fatal error: Call to a member function query() on a non-object ，
我应该已经实例化类了呀，为什么还是不能调用呢？

不说别的，从一开始就有问题
function _construct(){
  $this->mysqli_connect("localhost","root","","book");
mysqli_query($this->conn,"SET NAMES gbk");
}

mysqli_connect有定义么，

mysqli_connect（）不是连接数据库的函数吗，直接能用的呀？

哦，打错了，改成这个$this->conn=mysqli_connect("localhost","root","","book");不过还是同样的错误，求解决？

错误提示在这行上，红字体
function getrecordcount(){
$sql="select count(*) from content";
$result=$this->conn->query($sql); if($row=$result->fetch_row())
return (int)$row[0];
else
return 0;
}

function __construct(){
  $this->conn = mysqli_connect("localhost","root","","book");
  mysqli_query($this->conn,"SET NAMES gbk");
}

$this->conn 这个，从头到尾都买见你赋值过.........