Warning: mysqli_query() expects parameter 1 to be mysqli,Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result

I want to get single value from phpmyadmin database table but i am getting these two errors.I am working on a ecommerce website and in this code i am trying to get a quantity value from the cart table and save it on the local variable and then insert it on the Order detail table

                                   ***Connection***
<?php

$servername = "localhost";
$username = "root";
$password = "";  // set this field "" (empty quotes) if you have not set any password in mysql
$dbname = "ecommerce";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    //echo "Connection successful";
}
catch(PDOException $e)
    {
    echo $e->getMessage();
    }

?>
                                     ***Errors***  


 Warning: mysqli_query() expects parameter 1 to be mysqli
    Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result
                                  ***Code Where Error Exists***
$run_cart=mysqli_query($conn,"Select qty from cart vistor_id='$id'");
$get_qty=mysqli_fetch_array($run_cart);
echo $run_cart;
                                    ***All Code Here***


global $conn;
session_start();

        if(!isset($_SESSION['loggedin']))
            {
              echo "<script language=\"javascript\"type=\"text/javascript\">window.location.href=\"login.php\";</script>";
                exit(0);
        }
        $id =0.0;
            if(isset($_COOKIE['vistor_id']))
                {
                 $id=$_COOKIE['vistor_id'];
                }
        include("db/opendb.php");
        include("db/functions.php");
        $date=date('Y-m-d H:i:s');
        $run_cart=mysqli_query($conn,"Select qty from cart vistor_id='$id'");
        $get_qty=mysqli_fetch_array($run_cart);
        echo $run_cart;
        $msg = NULL;
        $dbobj = new Database();
        $dbobj1 = new Database();
        $query = "insert into order_t(order_des,order_date, fuser_name) values ('order placed at $date', '$date', '".$_SESSION['s_username']."')";
        $msg = $dbobj -> UpdateRecord($query, "Inserted");
        $orderid=$conn->lastInsertId();
        $query="select Product_id from cart where vistor_id='".$id."'"; 
        $dbobj -> FetchRecord($query);
        while($row = $dbobj->pstmt->fetch())
            {
              $query = "insert into order_detail(forder_id,fproduct_id,qty) values ('$orderid', '$row[0]','$qty')";
        $msg = $dbobj1 -> UpdateRecord($query, "Inserted");
        }
        $dbobj -> CloseDBCon();
        //echo "<script language=\"javascript\" type=\"text/javascript\">window.location.href=\"CheckoutComplete.php\";</script>";
        //exit(0);
        return