If your site is running on a LAMP (Linux, Apache, MySQL, PHP) platform it is possible to make the server execute PHP scripts inside of a .HTM or .HTML page. Pages with the HTML file extensions (.htm or .html) typically, when requested from the server are just sent straight to the browser by the server. With those extensions the web server assumes that there is nothing more to do than that. With Apache’s .htaccess file, however, you can alter this.
Why would you want to do that? It is common for webmasters to come across situations where a site consists entirely of static HTML files and the job does not allow for changing those extensions. Typically this is for search engine reasons.
The quick work-around for this is to add a line or to into the .htaccess file in the site’s root directory that will tell the server to look through each .html or .htm files for PHP scripting prior to sending them to the requesting browser/user. The exact lines of .htaccess code will depending upon the settings of the server that you’re running on but here are the most common.
AddHandler php-script .htm .html
AddType application/x-httpd-php .php .htm .html
Each simply tells the server to parse HTML pages as if they were PHP pages and generally, one of those two lines will accomplish the goal.