Use global variables in a class

I'm trying to create a pagination class and use a variable from outside the class.

But it's giving me the fatal error "Call to a member function query() on a non-object".

This is the index file:

$db = new DB_MySQL("localhost", "root", "", "test"); // connect to the database
include_once("pagi.php");

$pagination = new pagi();
$records = $pagination->get_records("SELECT * FROM `table`");

And this is the pagi.php file:

class pagi {

    public function get_records($q) {
        $x = $db->query($q);
        return $db->fetch($x);
    }

}

Is it possible to use this variable from out side of the class inside the class, without creating a new one inside the class?