Why I Cannot Reach An Array Splice Condition

    <?php
    require 'header.php';
    if (isset ($_POST['id'])) {
        $productid = $_POST['id'];
        $size = $_POST['size'];
        $wasfound = false;
        $i = 0;
        if (!isset ($_SESSION['cart']) || count($_SESSION['cart']) < 1) {
            $_SESSION['cart'] = array (0 => array ("product_id" => $productid, "size" => $size, "quantity" => 1));
        }
        else {
            foreach ($_SESSION['cart'] as $eachitem) {
                $i++;
                while (list ($key, $value) = each ($eachitem)) {
                    if (($key == "product_id" && $value == $productid) && ($key == "size" && $value == $size)) {
                        array_splice ($_SESSION['cart'], $i-1, 1, array (array ("product_id" => $productid, "size" => $size, "quantity" => $eachitem['quantity'] + 1)));
                        $wasfound = true;
                    }
                }
            }
            if ($wasfound == false) {
                array_push ($_SESSION['cart'], array ("product_id" => $productid, "size" => $size, "quantity" => 1));
            }
        }
        header ("location: cart.php");
        exit ();
    }
?>

Everytime I add the product_id same id or different id, same size or different size, it would be the new index on array. I cannot reach array_splice condition on the script. Anyone can help? Thanks in advance