How to create a global variable of type ?.class.php [PHP5]

[Shaitan00]

I'm trying to create a global variable ($db) of type Database (as shown below) to be accessed within Tools class.

I'm using PHP5 and running into a little problem... I have 2 files:
- Database.class.php
- Tools.class.php

The contents look as follows:

<?php
require 'Database.class.php';

class Tools
{
    public  $db       = "";	// is this the problem?

    function Tools($server, $user, $password, $database)
    {
        $db = new Database($server, $user, $password, $database);
    }
    
    public function Run()
    {
	$db->Connect();
    }
}
?>

<?php
class Database
{
    function Database($server, $user, $password, $database)
    {
        // DO SOMETHING HERE //
    }

    public function Connect()
    {
        // DO SOMETHING HERE //
    }

For some odd reason this doesn't seem to work ... When I am in Tools.class.php I am trying to access the Database public variable ($db) to run ($db->Connect()) as follows:
$db->Connect();
But ... as you may have guessed ... this doesn't work...

Any clues/hints would be much appreciated.
Thanks,