How to included a PHP file in an html page

I'm building a web page which has mostly static contents. Only one section of the page has to be generated based on the data from a text file. I wrote a PHP script to display that section. When I execute the script as a standalone script it displays the section correctly. Now I want to include this script in the html file .How do I do this?

I tried:
<?php include "head.php" ?>

where the dynamic page was to be displayed in the HTML file. But the page displays only the static contents. Is there any other way to execute the script from the html page?

what's the extension for this file? html? php? It must be php in order to execute the php code
Here's your file:
....
....
html code
....
....

<? include('head.php'); ?>
....
....
more html code
....
....