How to include header and footer of error document found in apache's custom error response package?

I have this line in my PHP script that includes a "file not found" 404 custom error provided by apache:

include('/var/www/error/HTTP_NOT_FOUND.html.var');
 

What is shown on my browser is just the body of the custom error page:

Content-language: cs Content-type: text/html; charset=ISO-8859-2
Body:----------cs-- Po?adovan? URL nebylo na tomto serveru nalezeno.
Zd? se, ?e odkaz na ">odkazuj?c? str?nce je chybn? nebo zastaral?.
Informujte, pros?m, autora

The error page contains the following comments:

<!--#include virtual="include/top.html" -->
 

and

<!--#include virtual="include/bottom.html" -->
 

which are supposed to be the header and footer found in the include folder. But this is outside of my document root, /var/www/html.

How do I include the header and footer without moving the error folder into the document root?

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