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Update shopping cart by adding quantities together if the same product is chosen

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This is the code that I'm using for a shopping cart in order to update the quantity if the same product id is chosen. 

$cart_items is a two-dimensional session array composed of the id session array and the quantity session array. While looping through $cart_items, I create a temporary array $temp_array so that I can execute some logic on the array without changing the normal output. $temp_array is the $cart_items array without the last items which would be the $_REQUEST id and the $_REQUEST quantity. So the two $_REQUESTs are popped off with array_pop and then the remaining $_SESSIONs are compared to the $_REQUEST id with 

$j = array_search($_REQUEST["element_id_$i"], $temp_array);
$j is now the key that corresponds to the id in $temp_array where the $_REQUEST id has been found. 

if( $j==$temp_array[$i]) 

If $j is the key of the item in $temp_array, then the $_REQUEST is unset and the quantities for the item are added together. 

There are two problems with this code. First of all the var_dump of the $temp_array is always an empty array but I am expecting to get an array with just the last element popped off. Secondly, 
I get the error "Undefined offset" for the $i variable in this line of the code: if( $j==$temp_array[$i]) 

foreach($cart_items as $item) {
foreach($item["id"] as $key => $val) {
foreach($item["quantity"] as $i => $value) {
    if ($key == $i){
$temp_array = $cart_items;
$array = array_pop($temp_array);

if (isset($_REQUEST["element_id_$i"]) && isset($_REQUEST["quantity_$i"])&& isset($value)){
$j = array_search($_REQUEST["element_id_$i"], $temp_array);
if( $j==$temp_array[$i]) {
echo "<b>SAME PRODUCT ADDED</b>";
$value += $_REQUEST["quantity_$i"];


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if you use the simple session cart definition that has been given on a different forum, using the item id as the array index, and storing the quantity as the array value under the index, doing what you are asking only takes a couple of lines of code.

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It's in mac_gyver's post that you just replied to ...

14 hours ago, mac_gyver said:

using the item id as the array index, and storing the quantity as the array value under the index

Do you not bother to read replies?

Here's a sample script to illustrate the point

$_SESSION['cart'] = $_SESSION['cart'] ?? [];

    $post = array_map('trim', $_POST);
    if (!empty($post['productid']) && !empty($post['qty'])) {
        if (!isset($_SESSION['cart'][$_POST['productid']])) {
            $_SESSION['cart'][$_POST['productid']] = 0;
        $_SESSION['cart'][$_POST['productid']] += $_POST['qty'];

<!DOCTYPE html>
<style type="text/css">
    body     { font-family: verdana, arial, sans-serif; font-size: 11pt; }
    fieldset { width: 400px; padding: 8px; }
    legend   {background-color: #000; color: #FFF; padding: 4px; }
    label    { display: inline-block; width: 100px; }
<?= '<pre>Cart = ' . print_r($_SESSION['cart'], 1) . '</pre>' ?>
    <form method="post">
        <fieldset style='padding: 30px; width:400px;'>
        <legend>Add product to cart</legend>
            <select name='productid' required>
               <option value=''>- Choose product -</option>
               <option value='PROD1'>Widget</option>
               <option value='PROD2'>Gizmo</option>
               <option value='PROD3'>Wotsit</option>
               <option value='PROD4'>Thingy</option>
            <input type="number" name="qty" value="1" size="5" required>
            <input type="submit" name="btnSub" value="Submit">



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