Hide return URL with Paypal IPN

Hello,

I want to hide the return URL on the link my script generate when it send users to Paypal.
The thing is I need to keep the parameter "userID" so when the user come back after payment my script will know which user to toggle on.
(userID is unique for each user)

this is the code for creating the payment link:

$gotourl = "https://www.paypal.com/cgi-bin/webscr?"
."cmd=_xclick-subscriptions"
."&business=MY_EMAIL@gmail.com"
."&item_name=ITEM NAME"
."&item_number=".$userID.""
."&no_shipping=1"
."&no_note=1"
."&currency_code=USD"
."&lc=US"
."&a3=".$fee.""
."&p3=1"
."&t3=Y"
."&src=1"
."&sra=1"
."&return=".urlencode($siteurl."/verify.php?verify=1")
."&cancel_return=".urlencode($siteurl)."";

Code (markup):

and when Paypal send the user back verify.php uses "userID" to update the user account to "active".

Your help will be much appreciated,
Thanks
Kobi