Call PHP script with loadVariables

I have a PHP-site/script called webshop.php with a $_SESSION variable where I store id’s of products.
I’ve tested the script via the browser with functions ?add_item=11 etc… Everything works at that side.

Now, I want to call this script from a flash-movie and thought this little code could do the job:
loadVariables("/webshop.php?add_item=" + id, this);

But it doesn’t work. (I’ve also tried with a hardcoded id just to test, webshop.php?add_item=11).

Anybody who can tell me what I do wrong?