PHP无法增加任何值，但最后添加到数组

I am using incrementing/decrementing to change the quantity demanded of items in my cart. However, this only seems to be working for the last value/row added to the array. e.g. if I add 1 item to the cart, incrementing ($_SESSION['cart'][$comic_id]++) works fine. However if I add a second item to the cart, incrementing only works for the latest added item and not the first added, and the same can be said for any additional added items.

Can someone please indicate where I have gone wrong and help me fix my code please?

I am echoing using the $value at the end contained within a foreach loop.

Here is my code (cart.php):

<?php 
if(!isset($_SESSION)) {
     session_start();
}
$total = 0;
$page = 'cart';

//Database connection info (Using MySQLi) removed for security

if(isset($_GET['comic_id'])){ 
    $comic_id = $_GET['comic_id'];
} else {
    $comic_id = NULL;}

$action = $_GET['action'];

if($comic_id && !productExists($comic_id)) {
    die("We apologise, but we can't seem to find the comic you were looking for!!!");
}

switch($action) {
    case "add":
        if (isset($_SESSION['cart'][$comic_id])) {
            $_SESSION['cart'][$comic_id]++;

        } else {
            $_SESSION['cart'][$comic_id] = 1;
            }
    break;  
    case "remove":
        if (isset($_SESSION['cart'][$comic_id])) {
            $_SESSION['cart'][$comic_id]--;}
        if($_SESSION['cart'][$comic_id] == 0) unset($_SESSION['cart'][$comic_id]);
    break;  
    case "empty":
        unset($_SESSION['cart']); 
        session_destroy();
    break;  
}

require('header.php');
require('sidebar.php');

if (isset($_SESSION['cart'][$comic_id])){

    echo "<table border=\"0\" padding=\"10\" width=\"80%\">";
    echo "<td colspan=\"1\" align=\"left\"><a href=\"title.php\">Continue Shopping</a></div>";
    echo "<td colspan=\"6\" align=\"right\"><a href=\"$_SERVER[PHP_SELF]?action=empty\" onclick=\"return confirm('Crystal Fusion: Are you sure you wish to empty your cart?');\">Empty Cart</a></td>";                      
    echo "<tr height=\"20px\">";
    echo "<tr height=\"20px\">";
    echo "<td align=center>Image</td><td align=center>Title</td><td align=center>Description</td><td colspan=3 align=center>Copies (+/-)</td><td align=center>Price</td>";
    echo "<tr height=\"20px\">";


    foreach($_SESSION['cart'] as $comic_id => $qty) {   

        $sql = sprintf("SELECT title, description, cost, image_thumbnail
                FROM comic 
                WHERE comic_id = %d;",$comic_id);

        $result = mysql_query($sql);

        if(mysql_num_rows($result) > 0) {

            list($name, $description, $price, $image_thumbnail) = mysql_fetch_row($result);

            $cost = $price * $qty; 
            $total = $total + $cost;

            $cost = number_format($cost,2);
            $total = number_format($total,2);
            $description =  substr($description, 0, 250);

            echo "<br><tr>";
            echo "<td width=\"10px\" align=\"center\"><img height=100 align=center src=\"$image_thumbnail\">";
            echo "<td align=\"center\">$name</td>";
            echo "<td width=\"40%\" align=\"center\">$description...<a href=comic_dyn.php?comic_id=$comic_id>More Info</td>";
            echo "<td width=\"30px\" align=\"center\"><a href=\"$_SERVER[PHP_SELF]?action=add&comic_id=$comic_id\">+<br></a><td align=\"center\">$qty <td width=\"20px\" align=\"center\"><a href=\"$_SERVER[PHP_SELF]?action=remove&comic_id=$comic_id\">-</a></td>";
            echo "<td align=\"right\">$$cost</td>";
            echo "</tr>";           
        }

    }

    echo "<br><tr><tr height=100px>";
    echo "<td><td><td colspan=\"4\" align=\"right\">Total:</td>";
    echo "<td width=\"60px\" align=\"right\">$$total</td>"; 
    echo "<tr><td colspan=\"7\" align=\"right\"><a href=\"checkout_html.php\">Proceed to Checkout</a>";
    echo "<tr height=\"50px\">";
    echo "</table>";
}else{

    echo "Your cart is currently empty.";
    echo "<br><br><td colspan=\"1\" align=\"left\"><a href=\"title.php\">Continue Shopping</a></div>";

}

$_SESSION['invoice'][$comic_id]=$name . " " . $qty . " " . $price;

foreach ($_SESSION['invoice'] as $value) {
    echo $value."<br>";}

function productExists($comic_id) {
        $sql = sprintf("SELECT * FROM comic WHERE comic_id = %d;", $comic_id);
    return mysql_num_rows(mysql_query($sql)) > 0;
    }
?>

you have to do foreach on your switch statement to add/remove the value from your each Items. Like that

foreach($_SESSION['cart'] as $comic_id => $qty) {
switch($action) {
    case "add":
        if (isset($_SESSION['cart'][$comic_id])) {
            $_SESSION['cart'][$comic_id]++;

        } else {
            $_SESSION['cart'][$comic_id] = 1;
            }
    break;  
    case "remove":
        if (isset($_SESSION['cart'][$comic_id])) {
            $_SESSION['cart'][$comic_id]--;}
        if($_SESSION['cart'][$comic_id] == 0) unset($_SESSION['cart'][$comic_id]);
    break;  
    case "empty":
        unset($_SESSION['cart']); 
        session_destroy();
    break;  
}


}