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OOP in PHP 4.4.2 vs 4.4.4

I am trying to re-write an old message board script I wrote awhile ago, using object oriented programming.

The problem is I keep getting errors on one server A using php 4.4.2 and it works fine on server B using php 4.4.4.

Anyone know what's wrong with my code and how/where I can find out how to fix it.

PHP Notice: Undefined variable: db in .../httpdocs/inc/messageboard.class.php on line 11
PHP Notice: Undefined variable: db in .../httpdocs/inc/messageboard.class.php on line 12
PHP Notice: Undefined variable: db in .../httpdocs/inc/messageboard.class.php on line 13
PHP Notice: Undefined variable: db in .../httpdocs/inc/messageboard.class.php on line 14

PHP:

<?php
include 'database.class.php';

class MessageBoard {

    var $db;
    var $tz_offset;
    var $cur_user_id;

    function MessageBoard ($current_user_id, $type, $host, $database, $user, $pass) {
        $this->$db = new database($type, $host, $database, $user, $pass); // line 11
        $this->$db->debug_mode(); // line 12
        $this->$db->query("SELECT timezone FROM fcms_users WHERE id = $current_user_id"); // line 13
        $row = $this->$db->get_row(); // line 14
        switch($row['timezone']) {
            case 'EDT': $this->$tz_offset = '-4 hours'; break;
            case 'CDT': $this->$tz_offset = '-5 hours'; break;
            case 'MDT': $this->$tz_offset = '-6 hours'; break;
            case 'PDT': $this->$tz_offset = '-7 hours'; break;
            case 'AKDT': $this->$tz_offset = '-8 hours'; break;
            case 'HST': $this->$tz_offset = '-9 hours'; break;
            default: $this->$tz_offset = '-4 hours'; break;
        }
    }

___________________

Haudenschilt.com

$this->db = new database($type, $host, $database, $user, $pass); // line 11
$this->db->debug_mode(); // line 12
$this->db->query("SELECT timezone FROM fcms_users WHERE id = $current_user_id"); // line 13
$row = $this->db->get_row(); // line 14

The above code should work. I've done a lot with OOP, but only a little with OOP in PHP, so I can't guarantee anything.