I have a webpage on a Joomla based website that I am trying to make inaccessible to anyone but those who have been redirected there through a redirect page.
Basically, they would purchase something on a form on my page which is integrated with Paypal, and when Paypal payment is complete I have them redirected to this specific page. I don't want anyone to be able to just copy this url and be able to come back to the page later. Is there a way to do this?
Then you have to store some "token" in your database, which is invalidated after the first time a returning client accesses your "thank you" page. The token should be given to the client (in the URL) when you redirect him to Paypal and when he comes back after a valid payment, the token must still be in URL. As far as I remember from the time I tried to use PayPal in one of my projects, it is possible to pass something like this to Paypal and get it back.
Checking a redirect is a poor way to do this. Such things can be easily spoofed. Instead have the pages you require a redirect from to create a session record of some kind and pass the ID of that record in the query string to the restricted page. The restricted page can deny if the session ID does not exist in the database or is too old.
It's been a while since I worked with Joomla. You might be able to look at the Server variables and check the Referring URL. If it's not one of your web pages or paypal, then you can redirect the user to the page of your choice.
I would include a specific token on the redirect. Maybe a Guid that has a limited life and is assigned to that specific user.
Paypal has a feature called Express Checkout that does just what you're looking for.
If you only care that the link expires shortly after it is used, then I think you should create a one-time URL for the content you are trying to protect. Have the action of purchasing create a unique key or token, store the value of the token on your server, and then reference that token in your link:
example.com/purchased_content?token=59803475203658902345089
When the link is clicked, check against your stored values. You can expire them based on whatever criteria you want.
If you also want to prevent copying the URL to somewhere else, you can have the action of purchasing set a cookie with the token value. Then check the cookie when the link is clicked. This is not foolproof since a knowledgeable user can copy the cookie too.