HTML is code the end user's browser will process and PHP is something that the webserver will process, including PHP in HTML pages isn't going to work and will just show the function in the page unless the server is instructed otherwise.
Assuming you're hosting in an Linux environment you can add the following to your .htaccess file which will instruct the server to execute PHP inside HTML and HTM pages.
AddType application/x-httpd-php .html
AddType application/x-httpd-php .htm
AddHandler x-httpd-php .html .htm .cgi .php
Your webhost will need support for PHP and .htaccess - please contact your host direct to address these issues.
Download html.zip for further help
About.com use this code. They also have a video tutorial on setting up PHP in HTML pages