I programming a ecommerce website where users can add products to a cart and then use a paypal checkout button to process the order. I've been trying to figure out how I would let the user add stuff to a cart and not force them to login to their account on my website till they try to checkout. I can't seem to figure it out because once you hit the paypal button it takes them to the paypal website, so I'm not sure where I would add a check to see if they are logged in or not and if not send them to the login page.
What you can do is to create a dummy check out button that will make them login, when they successfully logged in, then show them the paypal pay button or other payment button.
However, this may be make other shoppers to feel of being forced to register and logged in, so there should be an option for you to send this type of shoppers where they don't want to be bothered logging in or registering.
thats simple.. I know all these, because I once helped my older brother in developing extension modules for oscommerce and zen cart long time ago.. oscommerce and paypal sandbox was my training ground for php programming, and then one famous video site hired me as a non-contract developer, because I was a minor.
In fact, I think some people are still using the one and only contribution I have.. folding left category menu :). That was pretty weak, but I managed to contribute something at a very young age.. I don't even remember the name of the plugin anymore.. I need to look it up.. ;) that was long long time ago..