php - 更改与数据库的连接方式后清空 SESSION

Question

我正在创建一个购物车类，它以前在我使用全局 db 变量时工作，但现在在我在构造函数中建立连接后，get 变量变为空。这导致一个空篮子。

<?php
require_once 'config.class.php';
class cart
{
        private $db;

        public function __construct()
        {
            $this->db = new db();
            $this->db = $this->db->connect();
        }

            function writeShoppingCart() {
                $cart = $_SESSION['cart'];
                var_dump($cart);
                if (!$cart) {
                    return '<p>You have no items in your shopping cart</p>';
                } else {
                    // Parse the cart session variable
                    $items = explode(',',$cart);
                    $s = (count($items) > 1) ? 's':'';
                    return '<p>You have <a href="index.php?page=cart.php">'.count($items).' item'.$s.' in your shopping cart</a></p>';
                }
            }

            function showCart() {
                $cart = $_SESSION['cart'];
                if ($cart) {
                    $items = explode(',',$cart);
                    $contents = array();
                    foreach ($items as $item) {
                        $contents[$item] = (isset($contents[$item])) ? $contents[$item] + 1 : 1;
                    }
                    $output[] = '<form action="index.php?page=cart.php&action=update" method="post" id="cart">';
                    $output[] = '<table>';
                    foreach ($contents as $id=>$quantity) {
                        if($result = $this->db->prepare('SELECT * FROM book WHERE id = '.$id))
                        {
                            $result->execute();
                            $result->bind_result($id, $title, $author, $price);
                            $result->fetch();


                            $output[] = '<tr>';
                            $output[] = '<td><a href="index.php?page=cart.php?action=delete&id='.$id.'" class="r">Remove</a></td>';
                            $output[] = '<td>'.$title.' by '.$author.'</td>';
                            $output[] = '<td>&pound;'.$price.'</td>';
                            $output[] = '<td><input type="text" name="qty'.$id.'" value="'.$quantity.'" size="3" maxlength="3" /></td>';
                            $output[] = '<td>&pound;'.($price * $quantity).'</td>';
                            $total += $price * $quantity;
                            $output[] = '</tr>';
                        }

                    }
                    $output[] = '</table>';
                    $output[] = '<p>Grand total: <strong>&pound;'.$total.'</strong></p>';
                    $output[] = '<div><button type="submit">Update cart</button></div>';
                    $output[] = '</form>';
                } else {
                    $output[] = '<p>You shopping cart is empty.</p>';
                }
                return implode('',$output);
            }

                function addChart(){
                    $sql = 'SELECT * FROM book ORDER BY id';
                    $result = $this->db->prepare($sql);
                    $output[] = '<ul>';
                    while ($row = $result->fetch())
                    {
                        $output[] = '<li>"'.$row['title'].'" by '.$row['author'].': &pound;'.$row['price'].'<br /><a href="index.php?page=cart.php&action=add&id='.$row['id'].'">Add to cart</a></li>';
                    }
                    $output[] = '</ul>';
                    return implode('',$output);
                }
}
?>

<?php
session_start();
require_once('../resources/library/cart.class.php');
$obj_cart = new cart();
// Process actions
$cart = $_SESSION['cart'];
$action = $_GET['action'];
switch ($action) {
    //
    case 'add':
        if ($cart) {
            $cart .= ','.$_GET['id'];
        } else {
            $cart = $_GET['id'];
        }
        break;

    //in this case the foreach searches for product id which we arent deleting, and puts them in the $newcart
    case 'delete':
        if ($cart)
        {
            $items = explode(',',$cart);
            $newcart = '';
            //
            foreach ($items as $item) {
                if ($_GET['id'] != $item) {
                    if ($newcart != '') {
                        $newcart .= ','.$item;
                    } else {
                        $newcart = $item;
                    }
                }
            }
            $cart = $newcart;
        }
        break;
    //
    case 'update':
    if ($cart) {
        $newcart = '';
        foreach ($_POST as $key=>$value) {
            if (stristr($key,'qty')) {
                $id = str_replace('qty','',$key);
                if($newcart != '')
                {
                    $items = explode(',',$newcart);
                }
                else
                {
                    $items =  explode(',',$cart);
                }

                $items = ($newcart != '') ? explode(',',$newcart) : explode(',',$cart);
                $newcart = '';
                //
                foreach ($items as $item) {
                    if ($id != $item) {
                        if ($newcart != '') {
                            $newcart .= ','.$item;
                        } else {
                            $newcart = $item;
                        }
                    }
                }
                for ($i=1;$i<=$value;$i++) {
                    if ($newcart != '') {
                        $newcart .= ','.$id;
                    } else {
                        $newcart = $id;
                    }
                }
            }
        }
    }
    $cart = $newcart;
    break;


}

$_SESSION['cart'] = $cart;
$output[] = '<div id=\'shoppingcart\'>';

$output[] = '<h1>Your Shopping Cart</h1>';

$output[] = $obj_cart->writeShoppingCart();


$output[] = '</div>';

$output[] = '<div id=\'contents\'>';

$output[] = '<h1>Please check quantities...</h1>';


$output[] = $obj_cart->showCart();


$output[] = '<p><a href=\'index.php?page=add.php\'>Back to bookshop...</a></p>';


$output[] = '</div>';

echo implode($output);
?>

<?php
class db
{
    public $db = '';

            function connect()
            {
                try
                {
                    $this->db = new mysqli('localhost', 'root', '', 'webgame');
                }
                catch (Exception $e)
                {
                    echo 'cannot connect to database: ',  $e->getMessage(), "\n";
                }

                return $this->db;
            }
}
?>