i am using php server and want to use some php code in .html extention page

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.

Please refer this link[^] for more explanation.