Php Mysqli Global DB define for Classes ?

Question

How can i declare the DB Connection global so that every Class only use 1 config.php file ?

Atm i tried

global $db;
    define('DB_SERVER', 'localhost');
    define('DB_USERNAME', 'root');
    define('DB_PASSWORD', '');
    define('DB_DATABASE', 'DATABASe');
    $db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);    

    class login_class
        {

        function login($email,$pass)
            {

And of cause then in my Class

$db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);
                    $query=mysqli_query($db,
                    "
                      SELECT
                        *
                      FROM
                        users
                      WHERE
                        email='$email'
                        AND password = '$pass'
                     ");

                    while($row=mysqli_fetch_array($query,MYSQLI_ASSOC))
                    {
                        $userkey=$row['user_key'];
                        $password=$row['password'];
                    }

But this isnt working ...

I have 10+ clases which i use there so i prefer to have only 1 config File but all i try isnt working...