PHP Session variable not getting set

In one page of our site, I have this code:

$_SESSION['returnURL'] = "/store/checkout/onepage";

and further down, this button control:

<button type="button" title="Register Today" class="button" onclick="window.location = '/register/';" id="BecomeMember"><span><span>Become a Member Today</span></span></button>

Now, in the register template, I have this code:

<input type="hidden" name="returnURL" id="returnURL" value="<?php if(isset($_SESSION['returnURL'])) { echo $_SESSION['returnURL']; } else { echo '/'; } ?>" />

But it only shows the value as /.

What could be going on that is causing this?

Answer

What I ended up doing was sending a post variable to the page. The difference in the sessions between ExpressionEngine and Magento makes this prohibitive using session variables as well as cookies.