Is this by design?
On my Debian (Etch) server I noticed that Apache (2.2) will not send a Last-Modified header when serving PHP (mod_5.2.0) files (we’re talking plain-jane echo 'Hello World'; PHP files).
Additionally, I noticed that the header Accept-Ranges is also not served by Apache for PHP files, although my main conern for the moment is the Last-Modified header.
PHP are dynamic page that can change. So Apache can’t know the Last-Modified header.
You can add your own header with:
Of course if you need this header maybe you are looking to manage the request header If-Modified-Since too.
You can find it in
$_SERVER['HTTP_IF_MODIFIED_SINCE']