How to let your HTML page run PHP code?

Do you know that we can add PHP code to an HTML page, and run this PHP code? For the different PHP versions, we have to add different codes at .htaccess.
For php5, we add the following code:

# Use PHP5 as default
AddHandler application/x-httpd-php5 .html .htm .php

for php5.2, we add the following:

# Use PHP5 as default
AddHandler application/x-httpd-php52 .html .htm .php

same as others, say php5.3:

# Use PHP5 as default
AddHandler application/x-httpd-php53 .html .htm .php

Creat 2 page, demo.html, and insert the following code:

>

another page is php.inc, and insert the following code:

Visit demo.html, you will see:
well done

We also can add HTML code at php.inc.