数组不保存计算？

I have a simply cart array, that I want to update if the user selects the same product again. If they already have 5 pencils in their cart, and they add to cart 4 more pencils, I want them to have 9 pencils in their cart, not two entries for pencils of 5 and 4.

The code I have this:

$sub = array("id" => $_POST['pID'], "name" => $_POST['pName'], "price" => $_POST['pPrice'], "quantity" => (int) $_POST['pQuant']);
$updated = false;
foreach($cart as $c)
{
    if($c['id']==$_POST['pID']) {
        var_dump($c); 
        echo "<br>";

        $c['quantity'] += (int) $_POST['pQuant'];
        var_dump($c); 
        echo "<br>";
        var_dump($cart); 
        echo "<br>";
        $updated = true;
    }

}

if(!$updated) {$cart[] = $sub;}

For some stupid reason, (Im sure it's a really obvious thing i'm missing here), the first var_dump of $c is fine. That's the original values. Then the second var_dump, has the number added onto it. However on the third var_dump, of $cart, The value hasn't changed?

I don't get why.

Within the foreach the variable $c is only a copy of the basket item, so you update the copy, but not the original item within $cart. This can be circumvented by using a reference for $c instead:

foreach($cart as &$c)

Notice the & in front of $c.

foreach creates a copy of the original array. To work with the original array, use this:

foreach(array_keys($cart) as $k) {
    $c = &$cart[$k];
    // rest of code
}

The problem is, that foreach uses not references. It uses values. If you want to change $cart you have two ways:

foreach($cart as $key => $c) {
    $cart[$key]['quantity'] = += (int) $_POST['pQuant'];
}

The alternative:

foreach($cart as &$c) {// use a reference
[...]    
}