error

<?php
class Db{
    private $server   = 'localhost';
    private $userName = 'root';
    private $passwrod = '';
    private $select = 'catalog';
    private $charset = "utf8";
    private $sql = "";
    function __construct(){
        mysql_connect($this->server, $this->userName,$this->passwrod);
        mysql_select_db($this->select);
        mysql_set_charset($this->charset);
        $this->query($this->sql);
        $this->format($this->query($this->sql));
    }
    function query($sql){
        return mysql_query($sql);
    }
    function format($result){
        $array = array();
        while ($categories = mysql_fetch_array($result)) {
            $array[] = $categories;
        }
    return $array;
    }
}
$db = new Db();
$db->format($db->query("SELECT * FROM guest_book"));