Good day all, please how do i include a dynamic page(php) into an html page?

i did a part development for a website, and most of the pages are in html(but those part i did, i did in php).

meanwhile, there are some included file in my php that has to be global on all pages on the website.

so how do i incorporate these php files into the existing html files because renaming these html files is not an option.

i have tried the SSI method:
Code:
<!--#include file="myfile.php" -->
it did not work(even after renaming the file that contains the above code to a .shtml)

i also try including the PHP file using the hTML script tag:
Code:
<script src="myfile.php"></script>
can anybody please help?

Thanks