There are two methods shown here:-
http://www.wickham43.net/serversideincludes.shtml
The first paragraph is general and applies to both methods. The SHTML method is not used much now as PHP "includes" are much more popular and a PHP page can do lots of other things like web forms.
The PHP method means that you have several main pages and you insert common sections like a header or menu or footer into every main page with PHP code like
<?php include ("header.inc"); ?>
which links from just one file for each common section so that if a menu link changes, you only have one "include" file to change and it gets inserted in every other main page. The main pages with the php code must be given .php extension, not .html
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.