to convert the site to PHP, you will need to rename each of the HML files to change the extension from .htm to .php. that is really the only difference between a php file and an HTML file, the extension.
using the .php extension tells the php processor to look at the file, and if it finds any php code in it, process that code before sending the result to the client browser.
of course you will also need to edit links to the file so the links also use the .php extension.