How can I execute PHP code on my existing myfile.html page?

The way to execute PHP on a .html page is to modify your .htaccess file. This file may be hidden, so depending upon your FTP program you may have to modify some settings to see it. Then you just need to add this line for .html:

AddType application/x-httpd-php .html

Or for .htm

AddType application/x-httpd-php .htm

If you only plan on including the PHP on one page, it is better to setup this way:
AddType application/x-httpd-php .html
This code will only make the PHP executable on the yourpage.html file, and not on all of your html pages.