Help Needed implementing this simple Class

[bruin03]

I found a Modified Preorder Traversal Class here: http://www.aesthetic-theory.com/learn.php?mptt
Unfortunately, examples of the class weren't provided. I'm an OOP noob. Based on the class, could someone look at the class and tell me how to use it? I would really appreciate it. Thanks in advance!
//db structure

  CREATE TABLE `data` ( `id` MEDIUMINT UNSIGNED NOT NULL AUTO_INCREMENT, 
`title` VARCHAR(200) NOT NULL, 
`left_id` INT UNSIGNED NOT NULL, 
`right_id` INT UNSIGNED NOT NULL, 
`level` MEDIUMINT UNSIGNED NOT NULL, 
UNIQUE (`id`) );

<?php
class Tree

{

    var $data_array; // we'll store the data in this

    var $cnx; // our connection to the database



    // our constructer, we'll use it to connect to the database

    function Tree()

    {

        $cnx = mysql_connect(DB_HOST, DB_USERNAME, DB_PASSWORD) or die ('Unable to connect to database');

        mysql_select_db(DB_DATABASE, $cnx) or die ('Unable to select database');

    }



    /* this will display the data... we'll give it a few paramaters

     * parent: this will be an integer or false,

     *        if its an integer we will get all children,

     *        if its false we will get the entire tree

     * dig_levels: this will be an integer, or false

     *        represents the number of levels to dig BELOW PARENT for data.

     *        Examples, if its 3 and we get the entire tree, we'll get everything with a level <= 3

     *            (music,weezer,pinkerton BUT NOT elscorcho)

     *        ex: if its 2 and we have parent weezer (level:2) we'll get 2 + dig_level, or everything with a level <= 4

     * inc_parent: whether or not to include parent in result set

    */

    function get_data($parent = false, $dig_level = false, $include_parent = true)

    {

        if($parent)

        {

            $sql = 'SELECT `right_id`, `left_id`, `level` FROM ' . DATA_TABLE . ' ';



            $where = 'WHERE `id`=' . $parent . ';';

            $parent_res = mysql_query($sql . $where) or die(mysql_error());

            unset($sql, $where);

            if($include_parent) {

                $lt = '<=';

                $gt = '>=';

            }

            else

            {

                $lt = '<';

                $gt = '>';

            }

            $parent_arr = mysql_fetch_array($parent_res);

            //$parent_right = $parent_arr[0];

            //$parent_left = $parent_arr[1];

            //$parent_nest = $parent_arr[2];

            $where = 'WHERE `left_id` ' . $gt . ' ' . $parent_arr[1] . ' AND `right_id` ' . $lt . ' ' . $parent_arr[0] . ' ';

            unset($parent_res);

        }

        if($dig_level)

        {

            $nest_limit = isset($parent_arr) ? $parent_arr[2] + $dig_level : $dig_level;

            $level_adjustment = $nest_limit - $dig_level; // we'll use this later

            if(isset($where))

            {

                $where .= 'AND `level` <= ' . $nest_limit . ' ';

            }

            else

            {

                $where = 'WHERE `level` <= ' . $nest_limit . ' ';

            }

        }

        $sql = 'SELECT * FROM `' . DATA_TABLE . '` ' . $where . 'ORDER BY `left_id` ASC';

        unset($where, $parent_arr);

        $raw_result = mysql_query($sql) or die(mysql_error());

        while($item = mysql_fetch_array($raw_result)) {

            $data[$item['id']]['title'] = $item['title'];

            $data[$item['id']]['left_id'] = $item['left_id'];

            $data[$item['id']]['right_id'] = $item['right_id'];

            $data[$item['id']]['level'] = $item['level'];

            $data[$item['id']]['relative_level'] = $item['level'] - $level_adjustment;

        }

        unset($raw_result);

        $this->data_array = $data;

        return $data;
    }
}

?>