Need Help With Protected Link On Wordpress

[genuwine4532]

I have a Wordpress site, and on it there is are pages with download links that I want to limit access to via a special link that is only supplied to buyers, they are buying on an external site, and that site supplies them with download link.

 

I have a plugin where I could add a simple string to the end of the url (based on the plugins shortcode ?sap=code) and when I gave a link with that string at the end of the url it would take people to a page with the download link.

 

BUT

 

When if anyone went to the url or clicked a plain link to that page, it showed them some custom text I wrote, like Unauthorized and contact info.

 

But, this plugin stopped working in IE and now I need to do this manually, as I have found no other product to do this, been searching for hours.

 

Can I do this manually with PHP? Or is there a product like this?

 

Thank you!