objekt orienteret update db

Jeg har en fil (index.php) med en form der skal opdaterer en database....
i filen db.php findes mine klasser og funktioner

MEN jeg er helt ny i objektorienteret php og det virker ikke, kan du hjælpe?

index.php
<?
require('db.php');
$demo = new Example();
$list = $demo->getList();
$catid = $_POST["catid"];
$category = $_POST["category"];
$db = new Example();
$db->update_category($category,$catid);

    foreach($list as $item) {?>
    <div id="item_<?=$item['catid'];?>">
        <form action="<?=$_SERVER['PHP_SELF']?>" method="post">
        id=<?=$item['catid'];?> | orderid=<?=$item['orderid'];?>
        <input type="text" name="category" value="<?=$item['category'];?>" />
        <input type="hidden" name="catid" value="<?=$item['catid'];?>" />
        <input type="submit" name="submit" value="Update" /></form>
    </div>
<? }?>

db.php
<?
class Example {
    protected $conn;
    protected $user     = 'bruger';
    protected $pass     = 'kodeord';
    protected $dbname     = 'db navn';
    protected $host     = 'localhost';
   
    public function __construct() {
        $this->conn = mysql_connect($this->host, $this->user, $this->pass);
        mysql_select_db($this->dbname,$this->conn);
    }
   

    public function update($category,$catid){
            $this->result = mysql_query("UPDATE categories SET category={$category} WHERE catid={$catid}");
        }
   
   
    public function getList() {
        $sql = "SELECT * FROM categories ORDER BY orderid";
        $recordSet = mysql_query($sql,$this->conn);
        $results = array();
        while($row = mysql_fetch_assoc($recordSet)) {
            $results[] = $row;
        }
        return $results;
    }

}

nb
hverken
    public function update($category,$catid){
            $this->result = mysql_query("UPDATE categories SET category={$category} WHERE catid={$catid}");
        }
eller   
   
    public function update_category(){
        mysql_query("UPDATE categories SET category='$this->category' WHERE catid='$this->catid'");
    }
vil virke.........suk