First you have to edit the .htaxx file which should be located in the root of your web directory. If you don't have one, simply create one. Here's what you put in it.

i dont get it please explain

I think the original poster meant the .htaccess file (note the leading dot) in the root directory not the .htaxx file.

Basically what you are doing is saying to the server (Linux only not Windows) that if it finds php code in a file with an html or htm extension that it should pass the code to the php interpreter for processing. If you don't have the above then the majority of servers will only parse php code in a page that ends in .php / .php3 / .php4

To explain the code in the .htacess file you are removing the handler for .html and .htm from the webserver and then adding them to the php parser bit of the webserver.

Cheers