Prevent cross-domain include in PHP

Prevent cross-domain include in PHP

I imagine this question has already been asked but I can't find it, so I
am sorry for the eventual duplicate. I have only found the opposite.
How do I prevent cross domain includes in PHP? Is a preg_match of the
$_SERVER["HTTP_REFERER"] enough? My guess is no. What is the option, if
any, on the php.ini to prevent this?
Thank you.