Menampilkan data secara oop menggunakan mysqli

code ini kok gak bisa nampilin data ya ? apa sudah benar code untuk menampilkan data oop dengan mysqli seperti berikut :

index.php

<?php
$res = $crud->read();
if(mysql_num_rows($res)>0)
{
	while($row = mysql_fetch_array($res))
	{
	?>
    <tr>
    <td><?php echo $row['first_name']; ?></td>
    <td><?php echo $row['last_name']; ?></td>
    <td><?php echo $row['user_city']; ?></td>
    <td><a href="edit_records.php?edt_id=<?php echo $row['user_id']; ?>">edit</a></td>
    <td><a href="dbcrud.php?del_id=<?php echo $row['user_id']; ?>">delete</a></td>
    </tr>
    <?php
	}
}
else
{
	?><tr><td colspan="5">Nothing here... add some new</td></tr><?php
}
?>

class.crud.php

<?php
include_once 'dbconfig.php';

class CRUD
{
	public function __construct()
	{
		$db = new DB_con();

	}

	public function read()
	{
		global $db;

		$qru = "SELECT * FROM users ORDER BY user_id ASC";
		$result = mysqli_query($db, $qru);
		return $result;
	}
?>

dbconfig.php

 <?php
define('DB_SERVER','localhost');
define('DB_USER','root');
define('DB_PASSWORD','');
define('DB_NAME','test');

class DB_con
{
	function __construct()
	{
		$conn = mysqli_connect(DB_SERVER,DB_USER,DB_PASSWORD) or die('error connecting to server'.mysqli_error());
		mysqli_select_db(DB_NAME, $conn) or die('error connecting to database->'.mysqli_error());
	}
}

?>