PHP PDO - cannot serialize or unserialize PDO instances

Question

I need to pass a PDO connection into a cart class from a controller,

function __construct($connection) 
{
    $this->cart = new cart($connection);
}

but I think the problem is with serialize()

public function render_page() 
{

    if (!isset($_SESSION[SESSION_CART]))
    {
       $cart =  $this->cart;
    }
    else 
    {
       $cart = unserialize($_SESSION[SESSION_CART]);
    }

    $_SESSION[SESSION_CART] = serialize($cart); 

 }

I get this error,

Fatal error: Uncaught exception 'PDOException' with message 'You cannot serialize or unserialize PDO instances' in C:\wamp\www\store_2012_MVC\local\controllers\class_base_extended_cart.php:89 Stack trace: #0 [internal function]: PDO->__sleep() #1 C:\wamp\www\store_2012_MVC\local\controllers\class_base_extended_cart.php(89): serialize(Object(cart)) #2 C:\wamp\www\store_2012_MVC\local\controllers\class_factory.php(75): base_extended_cart->render_page() #3 C:\wamp\www\store_2012_MVC\index.php(69): factory->render() #4 {main} thrown in C:\wamp\www\store_2012_MVC\local\controllers\class_base_extended_cart.php on line 89

How can I fix this?

Or can I use something else instead of serialize()?

EDIT:

I tried it with __sleep and __wakeup magic methods but still get the same error,

class database_pdo
{
    # database handler
    protected $connection = null;

    # make a connection
    public function __construct($dsn,$username,$password)
    {
        try 
        {

            $this->connection = new PDO($dsn, $username, $password, array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8"));
            $this->connection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 

        }
        catch (PDOException $e) 
        {
            # call the get_error function
            $this->get_error($e);
        }
    }

    # don't forget to add getter method to get $this->connection, it's just a good practice.
    public function get_connection()
    {
        return $this->connection;
    }

    public function __sleep()
    {
        return array('connection');
    }

    public function __wakeup()
    {
        $this->connection;
    }

}

Answer 1

PDO objects contain active links to databases (which may have a transaction initiated or db session settings and variabiles).

You cannot serialize a PDO object because the above would get lost and cannot be re-established automatically.

You should redesign your classes to access the current database link statically using a separate class (dedicated for holding db connections), instead of saving a reference in a member variable (I supose this is happening when you do new cart($connection))).