How can I restrict access to a php file so that it can only be reached via iframe in my server?

I have a content.php file on my server that I need to load in an iframe on another page mask.php. This works just fine but I do not want content.php to be reachable at mydomain.com/content.php, only at mydomain.com/mask.php via iframe.

Is there a way to restrict the access to content.php to only via iframe on same server? PHP or HTTP?

I read about restricting the referer of content.php, so that it has to be mask.php for it to load but it doesn't seam to work. I can imagine that it should be doable with PHP.

Thanks for any ideas!