PHP Unable to increment any value but the last added to the array
<p>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. </p>

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

<p>I am echoing using the $value at the end contained within a foreach loop.</p>

<p>Here is my code (cart.php):</p>

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


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

    }

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

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

}

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

foreach ($_SESSION['invoice'] as $value) {
    echo $value."&lt;br&gt;";}

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