PHP Object Extension Question

So I have an item class as follows:

class Item
{
    private $db;
    private $data = array(
        'AltItem1' => null,
        'AltItem2' => null,
        'BaseUOM' => null,
        'Category1' => null,
        'Category2' => null,
        'Category3' => null,
        'Category4' => null,
        'Iden' => null,
        'IsHCS' => null,
        'ItemDesc' => null,
        'ItemNmbr' => null,
        'ItemType' => null,
        'MSDS' => null,
        'NoteText' => null,
        'NonStock' => null,
        'PrcLevel' => null,
        'TipPrice' => null,
        'DTM_UpdType' => null,
        'DTM_UpdDateTime' => null,
        'DTM_DownloadDateTime' => null,
        'DTM_UploadDateTime' => null
    );

    public function __construct(mysqli $db, $id = null){
        $this->db = $db;

        if(!empty($id)){
            $id = (int)$id;
            $this->populate($id);
        }
    }

    public function __get($key)
    {
        if(array_key_exists($key, $this->data)){
            return $this->data[$key];
        }
        error_log("Invalid key '$key'");
        return null;
    }

    public function __set($key, $value)
    {
        if(array_key_exists($key, $this->data)){
            $this->data[$key] = $value;
            return true;
        }
        return false;
    }

    public function populate($id)
    {
        $sql = sprintf(
            "SELECT %s FROM ItemMaster WHERE id = ?",
            implode(", ", array_keys($this->data))
        );

        $stmt = $this->db->stmt_init();
        $stmt->prepare($sql) or die ("Could not prepare statement:" . $stmt->error);
        $stmt->bind_param('i', $id);
        $stmt->execute() or die('exec');
        $stmt->store_result();
        if($stmt->num_rows == 1)
        {
            $params = array();
            foreach($this->data as $key => $val){
                $params[] = &$this->data[$key];
            }

            call_user_func_array(array($stmt, 'bind_result'), $params);
            $stmt->fetch();
            $return = true;
        }
        else{
            user_error("No rows returned for id '$id'");
            $return = false;
        }
        return $return;
    }
    public function insert()
    {   
        $params = $this->data;
        $values = array();

        foreach($params as $param){
            $values[] = "?";
        }

        $sql = sprintf(
            "INSERT INTO recurrence (%s) VALUES (%s)",
            implode(", ", array_keys($params)),
            implode(", ", $values)
        );

        $stmt = $this->db->stmt_init();
        $stmt->prepare($sql) or die ("Could not prepare statement:" . $stmt->error);

        $types = str_repeat("s", count($params));
        array_unshift($params, $types);
        call_user_func_array(array($stmt, "bind_param"), $params);

        $stmt->execute();

        $stmt->store_result();
        $result = $stmt->result_metadata();
    }
    public function update()
    {
        $sql = "UPDATE recurrence SET ";
        $params = array();
        foreach($this->data as $key => $value){
            $params[] = "$key = ?";
        }
        $sql .= implode(", ", $params) . " WHERE id = ?";

        $stmt = $this->db->stmt_init();
        $stmt->prepare($sql) or die ("Could not prepare statement:" . $stmt->error);

        $params = $this->data;
        $params[] = $this->data['id'];
        $types = str_repeat("s", count($params));
        array_unshift($params, $types);
        call_user_func_array(array($stmt, "bind_param"), $params);

        $stmt->execute();

        $stmt->store_result();
        $result = $stmt->result_metadata();
    } 

    }

My question is what would be the best way to extend this class with the data structure the way I have it? I basically want another class for an item in a shopping cart. So some extra fields would be quantity, cart id, etc. Or is there a better way to do this without extending the class?

On another note, say I have another variable $price thats not store directly in the database. So I make it a public variable, but I would have to make helper methods to access it wouldn't I? If that's the case, is my $data array the best solution of this type of item?

Thanks in advance.

I'm not 100% sure what kind of usage of the private $data variable you might be doing, so my tendency here would be to take a slightly different approach.

Instead of grouping all your data fields inside a single private variable of the object, I would make each field a private variable itself, ie:

class Item
{
    private $db;
    private $AltItem1;
    private $AltItem2;
...
etc.

This would immediately solve your problem with having publicly available data fields as well, as you could simply declare such fields as a public member. Public members don't require a getter and a setter, so you wouldn't have to worry about that... you could just access them through $this->price (internally), or $item->price (externally). Saves you some code. And it would be a quick modification of your populate() function to set all your new properties, as all you'd have to do would be to set $this->$$key instead of $this->data[$key].

Now, with your use of __set() and __get(), it looks like you want to be able to access the private $data member even from outside the object. There's no reason you can't continue that by having each field declared separately private as well. __set() and __get() will operate exactly the same way, you'd just need a minor adjustment, ie:

public function __get($varname)
{
    if ($this->$varname !== null) return $this->varname;
    error_log("Invalid key '$key'");
    return null;
}

As a final bonus, extending the class becomes easier, because you don't have to redeclare all the fields in their entirety if you want to override the $data property. You simply add the new fields of your children as new private members.

So I'm not sure if that makes your life easier, but I think that would be my approach.