Adapting Session Variables

[presta-pants]

Hi there!
I have a jquery popup window that I have installed on my client's PrestaShop store, however the php variable that I would normally use to make sure it doesn't load every time for a user won't work with PrestaShop...I'm not sure quite how to adapt it.
I'm very new to PrestaShop, although not that new to PHP, so not sure which files to configure to make this work.

Here is a mashup of the code I intended to use:

<code>

<?php
session_start();
?>

<?php
    if(!isset($_SESSION['jquery_popup']))  {
    $_SESSION['jquery_popup'] = 1;
?>
<?php }
    //if((isset($_SESSION['jquery_popup'])))  { unset($_SESSION['jquery_popup']); } //uncomment for testing
    ?>
</code>

 

Thanks so much!