i want to call my script receive.php after payment successfull in automatic.

i do:

$username='user';

<form action="https://www.sandbox.paypal.com/it/cgi-bin/webscr" method="post">
...
<input type="hidden" name="notify_url" value="www.mysite.com/receive.php?username=$user">
...
<input type="image" src="http://www.sandbox.paypal.com/it_IT/i/btn/x-click-but01.gif" name="submit">
</form>

i added to my sandbox paypal account business this address for ipn:
www.mysite.com/receive.php

why it don't work?