Hello @kartik,
You can't run PHP in .html files because the server does not recognize that as a valid PHP extension unless you tell it to. To do this you need to create a .htaccess file in your root web directory and add this line to it:
AddType application/x-httpd-php .htm .html
This will tell Apache to process files with a .htm or .html file extension as PHP files.
Hope it helps!!