How to add last-modified to a site where everything is on files?

Question

htaccess

Hi, I came across a site here where everything is roughly speaking on html files (php, but from php there is only include and that's it)
The architecture is as follows:
for example, we turn to the site/contacts.php page
at the root there is a contacts.php file and there it includes a header and footer
How can I add a Last Modified header to the response, and record the last edit of the file there, so that I do not have to insert the last edit definition code into each page (file)?
I tried to insert in htaccess: I
started giving 404 at all

RewriteRule .* - [E=HTTP_IF_MODIFIED_SINCE:%{HTTP:If-Modified-Since}]
RewriteRule .* - [E=HTTP_IF_NONE_MATCH:%{HTTP:If-None-Match}]

Please tell me, I'm sure this can be somehow solved with little bloodshed)