You can make a lay-out page and just put this code where you want the content to be and the content will be the only thing that will chnage.
This is how you link to it:
(page of the code)?id=(filename of the include file)
ex:
layoutpage.php?id=content
or
layoutpage.php?id=content.php
PHP Example :
<?php
if(file_exists("$id.html")){
include ("$id.html");
} elseif (file_exists("$id.php")){
include ("$id.php");
} elseif (file_exists("$id.txt")){
include ("$id.txt");
} elseif (file_exists("$id")){
include ("$id");
} else{
include ("file could not be include");
}
?>