Need Help for Paypal.class.php

problem with sandbox..ipn validation failed..

at a quick glance i would say that the line

if(!isset($_SESSION["userlogin"])) { header("location: login.php"); exit(); }

would be stopping the paypal system from connecting to the script

Hmm. so please advice what should i do for this...
but when user is click on payment he successfully go to paypal website and when it returned back my deposti_init.php take him to succesful deposit command.
but what i fel Command from Paypal returned for deposit_init.php?action=ipn is not initializing or maybe some error or missing something in this part?