How to let your html page run php code?

Do you know that we can add php code at a html page, and run these php code? For different php version, we have to add different code 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:

Let .html page run php 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.