To parse PHP code inside a .html file you will need to a create a .htaccess file in your document root directory /httpdocs and insert the following code:

RemoveHandler .html .htm
AddType application/x-httpd-php .php .htm .html

You will need to ensure that PHP is running as Apache, to check  this in Plesk you will need to login to your client account, select the domain name and go to Web hosting settings.