a .htm page that gets content from php.
 

Technically, this can easily be done with a .htaccess file. You don't even need mod_rewrite. Just say HTML files are PHP.

If that is all, then you can add

AddType application/x-httpd-php .html

to your .htaccess file.

This tells the server to parse your html files for php. Then simply place some php code within your page as you would a normal php file and it will be parsed.