Code required to enter website PHP

Very simple project. I wish to only allow visitors to access my website by either paying a fee via PayPal or by entering a valid 4-digit code. See a mockup of my homepage here: [login to view URL] Visitors accessing the site via PayPal shall access page 2 of the site automatically when a successful payment has been processed. I will set this function up through my PayPal account. When a valid code is entered and the submit button is clicked in order to gain access, the visitor shall access page 2 of the site. When page 2 is rendered, the visitor shall have the option of clicking on a button to be forwarded to an affiliate page that uses the same 4-digit code as the one that was entered. If an invalid code is entered, the visitor is returned to the homepage. See a mockup of page 2 here: [login to view URL] (On this page, clicking the button below the video shall render the affiliate page.) For example: The visitor entered "5737" and clicks "submit" to access the site. The number "5737" shall automatically become a part of the target URL for the affiliate webpage button on page 2. ex: ([login to view URL]) I shall have access to an admin area where I can add codes for my affiliates. The codes I enter in the admin area shall then be seen by the system as valid codes. All codes shall be 4-digit, numeric.