>I'm developing an web app where the customer will use pay pal to pay for screen savers. I want to make sure that they have arrived at my download page from the pay pal page.
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>Is there any way to check a visitor's web browser to see if it was just sent to the page from a paypal.com address? I.e... can I get the last url??? or something that will let me know where they came from?

You cannot rely on the HTTP_REFERER as, first some users are using 3rd party tools that are hidding that variable, secondly, because someone who would type directly in the URL would make it blank.

You would have to rely on an internal mechanism such as passing a key that only your system can recognize. So, that could be a sign of the source of your previous page.