The include() function takes all the content in a specified file and includes it in the current file. Example:

<div id="nav">
<?php 
include("menu.php"); 
?>
</div>

The above example demonstrates how a <div> tag in one page includes the contents of another file (menu.php).   The include() function allows you to share one menu file between multiple web pages.

http://www.w3schools.com/php/php_includes.asp