更新会话变量不起作用php

Question

im now working in a cart where all products saved in a session variable. So if the same product exists in the cart it will just increase the quantity. But it looks like working when i echo out the variable but when i print the session variable it remain same. here is my code

<?php
require_once("inc/init.php");

$product_id = htmlentities($_POST['product_id'], ENT_QUOTES);
$quantiy_added = htmlentities($_POST['quantiy_added'], ENT_QUOTES);
$op = htmlentities($_POST['op'], ENT_QUOTES);

$Cart = new Cart();

//var_dump($Cart);

global $mysqli;

if ($op == "add-item") {
    if (isset($_SESSION['careat_cart'])) {              //if same item exists
        foreach ($_SESSION['careat_cart'] as $key => $value) {
            if ($product_id == $value['id']) {
                $value['quantity'] += $quantiy_added;
                echo $value['quantity'];
            } else echo "new item";

        }

    }
} 

Answer 1

<?php
session_start();
require_once("inc/init.php");

$product_id = htmlentities($_POST['product_id'], ENT_QUOTES);
$quantiy_added = htmlentities($_POST['quantiy_added'], ENT_QUOTES);
$op = htmlentities($_POST['op'], ENT_QUOTES);

$Cart = new Cart();

//var_dump($Cart);

global $mysqli;

if ($op == "add-item") {
    if (isset($_SESSION['careat_cart'])) {              //if same item exists
        foreach ($_SESSION['careat_cart'] as $key => $value) {
            if ($product_id == $value['id']) {

                $value['quantity'] += $quantiy_added;
                echo $value['quantity'];
            } else echo "new item";

        }

    }
} 

Answer 2

I found the solution. It would be

$_SESSION['careat_cart'][$key]['quantity']+= $quantiy_added;

Instead of

$value['quantity'] += $quantiy_added;