PHP Redirect on WordPress Site

General information for the website: File download restriction
Description of requirements/features: I have a website that allows people to download files. Due to medical laws, users have to confirm that they are a medical professional. I currently have a popup etc that confirms all of this. I have also installed Simple Download Monitor to mask the file url. The problem I now have is that if a user shares the masked url with someone else they are able to download the file. My aim is to redirect users if they did not come from another page on our website.

The download URL is formatted as such:
http://[domain]/?smd_process_download=1&download_id=628

I have the following code:
$referrer = $_SERVER['HTTP_REFERER'];
$actual_link = "$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";

echo 'Referrer: '.$referrer.'';
echo 'Actual Link: '.$actual_link;
if ((strpos($referrer, '[domain]') !== false) && (strpos($actual_link, 'smd_process_download') !== false)) {
// true
} else {
// false
header('Location: http://google.com');
}

The aim is that if the referrer url does not contain our domain and the url does contain the smd_process_download string then to redirect the user to the homepage.

Currently the code redirects any page request to Google which obviously isn't useful!

What have a missed within this and is there a better way to do this?

Thanks in advance!
Extra notes: