PDO connection bind value not working

Question

I'm not what is the issue with this code, i'm trying to bind value but it is not working properly. I'm getting

INSERT INTO menucategory (menu_type,menu_order,menu_status) VALUES (?,?,?)

as it is

Here is my code

<?php 

class RDBConnection{
private $host,$user,$password,$dbname;
public $dbh,$sth;
function __construct(){
   $this->host="localhost";
   $this->user="user";
   $this->password="pass";
   $this->dbname="zczc";
}
// connect method create connection
function connect(){
    try{
  $this->dbh=new PDO("mysql:host=$this->host;dbname=$this->dbname",$this->user,$this->password);
  // set the PDO error mode to exception
  //$this->dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }
    catch (PDOException $e){
        echo $e->getMessage();
    }
}
// disconnect method disconnect connection created
function disconnect(){
    $this->dbh=null;
}
// insert
function inserMenu($menu){
    $this->sth=$this->dbh->prepare("INSERT INTO menucategory (menu_type,menu_order,menu_status) VALUES (?,?,?)");
    $this->sth->bindParam(1, $menu[0]);
    $this->sth->bindParam(2, $menu[1]);
    $this->sth->bindParam(2, $menu[2]);
    $this->sth->execute();
   return true;
}
}
?>

I referred it from here http://php.net/manual/en/pdostatement.bindparam.php#118975