<!-- html -->
<form action="cart.php" method="POST">
    <input type="hidden" name="product_id" value="<?=product_id">
    <input type="number" name="qty">
    <button type="submit">Add To Cart</button>
</form>

<!-- cart.php -->
session_start();
if (!empty($_POST['product_id'], $_POST['qty'])) {
    $product_id = $_POST['product_id']; // please validate input
    $qty = $_POST['qty']; // please validate input
    $_SESSION['cart'][$product_id] = $qty;
    exit(header('location: ' . $_SERVER["HTTP_REFERER"]));
}

<!-- viewcart.php -->
<?php
session_start();
foreach ($_SESSION['cart'] as $product_id => $qty) {
    echo 'Product Id: ' . $product_id . ' & Quantity: ' . $qty;
}
?>