Usually when we run PHP, we put it in a file with a .php extension. Sometimes however, we need to execute PHP code on a .html page. The easiest way would be to simply rename the page from yourfile.html to yourfile.php. This however can cause problems if the page has been around awhile and has lots of incoming links, or is already ranked in search engines, and you don't want to change it's name.
The solution is to modify the .htaccess file
to make PHP executable on .html files. By doing this you can make all your .html/.htm files run PHP, or you can set it to simply process one specific page (yourpage.html for example) as PHP.
Some hosting companies also allow you to change this setting from your control panel, or if you contact them they may be able to change this setting for you.