'Content-Type' is 'text/html' (not for other content types).
Please check out this topic to overcome this issue.
The header 'Last-Modified' is removed from response both for static *.html files
Not true. Please browse your HTML file, press F12, go to
Network tab, press on request and check again
and for content generated with PHP
PHP scripts are used to generate dynamic content. Holding a "Last-Modified" date would be useless for them..