Web based captive portal (no login required) with page redirects

I want to create a webpage that only allows a redirect to another webpage if the account is up to date on payments. (I don't want anyone to have to login at this point). I think it will work something like this flowchart...

1. User goes to website i.e, www.mobiletechbay.com
   
2. then that landing page will check against some sort of payment gateway (hopefully PAYPAL) to see if the website is up to date on payment.
   
   2a) if payments are up to date, then it redirects to a subdomain like www.tech1.mobiletechbay.com. I think that some sort of POST information will need to be passed to allow access to the subdomain.
   
   2b) if payment is not up to date, then it redirects the user to payment gateway to pay for activation.
   I want the landing page to do all the work to check if account associated to it is actually paid up and then redirect accordingly to the two options of 1) being paid up or 2) not being paid up. Does anyone have any ideas...or know of a program that already does this? The key for me is to not have anyone have to authenticate to the landing page unless payment is required. Any help will be greatly appreciated. THANKS!