How to use php file as an errorpage?

I currently have custom html as an errorpage, but would like to use php.

Errorpages are configured in /etc/nginx/sites-enabled/mysite.com.conf

Apparently just changing
error_page 50x = /errorfiles/error50x.html;
to
error_page 50x = /errorfiles/error50x.php;
doesn't work.

What should I do to make my php errorpage work?

The reason I want to do this is that I want to send an email to myself when error 50x happens. Flooding my emailbox is not an issue here.