将$ variable从一个php传递到另一个php

Question

How could I send $quantity from cart.php and place it into index.php?

I've tried this:

At index.php:

<?
session_start();
$quantity = $_SESSION['sessionVar'];
?>

<span><?php echo $quantity ;?></span>

At cart.php:

<?php 
session_start();
$quantity = $_POST['quantity'];
$_SESSION['sessionVar'] = $quantity;
?>

Would you suggest any solution here? Thanks!