How do I only allow visitors from a certain domain see a page on my website?

e.g. I want my page www.example.com/page-7 to only be visible to visitors coming from www.referral.com and show nothing/an error for everyone else

 

On the example.com/page-7, add to the top:

$domain = false;
if (isset($_SERVER['HTTP_REFERER'])) {
     $domain = parse_url($_SERVER['HTTP_REFERER'],  PHP_URL_HOST);  
}
if ($domain != 'referral.com' && $domain != 'www.referral.com') {
    die('Invalid redirect');
}

PHP:

However, you have to keep in mind that the HTTP_REFERER can easily be faked via headers. A good way, if you own both domains, is to redirect with a custom header that contains a token known to both sides. It won't make it fool proof, but would deter most of scrapers. A fool proof way would be to use shared db to store the sessions when links are clicked via a proxy page on your site: ex: referral.com/proxy.php which adds a custom token to the db and passed it via a GET param to your site, which your site checks the DB and then deletes it.

 

Here is a shorter version if you prefer this style of coding:

if (! in_array(parse_url(@$_SERVER['HTTP_REFERER'], PHP_URL_HOST), ['referral.com', 'www.referral.com'])) die("Not allowed");

PHP:

(requires PHP 5.4)