So you want people to be able to add text into a static HTML file?
Something like a comment system you might find on a blog.
The simplest way would probably be to have a HTML file that contains a placeholder of sorts, where you want new lines to be added.
Then you could load the entire file into your PHP code and replace that placeholder with the new line, adding a new placeholder after that line, and finally overriding the old HTML file with the new one.
That wouldn't be hard to implement, and would just require basic knowledge of PHP.
Look into the
file_get_contents,
str_replace and
file_put_contents functions if you want to try it out.
That is, obviously, not the best way to do this, but is is the simplest.
Other, more refined methods, would involve some sort of a database, and a PHP code that would assemble the page on each request (or possibly create a cached file... but that's a while another issue).