I have a .php page I don't want anyone being able to just open it unless
they've been through a previous page that is a disclaimer http referrer etc
come to mind but not exactly sure how to implement it in PHP.
Yes, you could check the value of $_SERVER['HTTP_REFERRER'] but that's not perfect. Instead, you could have the previous page contain a form with a special hidden input. The form is submitted to the protected page, where you check for that hidden input's value. If that protected page doesn't receive $_POST data containing whatever meaningful data, redirect them accordingly.
Hope that helps,
Larry