PHP/MySQLi - 致命错误：在非对象上调用成员函数 mysqli_query（）

问：

这是我的代码：

require "../include/functions.php";

error_reporting(E_ALL);
ini_set('display_errors', '1'); 

ConnectWithMySQLiDatabase();

$Cat = addslashes($_POST["Category"]);

$v = $conn->mysqli_query($conn,"SELECT * FROM `categories` WHERE `id`=$Cat");
$vrowi = mysqli_fetch_array($v, MYSQLI_ASSOC);

$url = $conn->real_escape_string($vrowi['Link']);

这是我所拥有的：functions.php

function ConnectWithMySQLiDatabase() {

     global $dbhost, $dbuser, $dbpass, $database, $HTTP_SERVER_VARS;

    $conn = new mysqli($dbhost, $dbuser, $dbpass, $database);
    // Check connection
    if (mysqli_connect_errno())
      {
      echo "Failed to connect to MySQL: " . mysqli_connect_error();
      }


    $conn->set_charset("utf8");

    global $conn;

}

变量设置正确。$dbhost, $dbuser, $dbpass, $database,

当我尝试执行此mysqli_query时，我收到以下错误：

<b>Fatal error</b>:  Call to a member function mysqli_query() on a non-object in <b>/fetch_category_products.php</b> on line <b>19</b><br />

第 19 行是：

$v = $conn->mysqli_query($conn,"SELECT * FROM `categories` WHERE `id`=$Cat");

你能告诉我我的错误在哪里，我该如何解决吗？

提前致谢！