PHP Include Function
From the menu go to: Insert > PHP Tags >Include.

The include() function allows you to access an external file of code inside your application. Using includes() is a great way to break your site into more manageable files and to also change values across your entire site using variables. For example, a repetitive header... Changing one file updates your entire Web site!

Example, a simple navigation system:

navigate.inc
<p>
<u>Navigate</u><br>
<a href="page1.htm">Page1</a><br>
<a href="page2.htm">Page2</a><br>
<a href="page3.htm">Page3</a><br>
<a href="page4.htm">Page4</a><br>
</p>

yourpage.htm
<html>
<head>
<title> PHP Include() example </title>
<meta name="Generator" content="AlleyCode">
</head>

<body>

<table width="100%" border="1" cellspacing="4" cellpadding="4">
<tr>
<td width="100"><? include "$DOCUMENT_ROOT/include/navigate.inc"; ?></td>
<td>Body content of your web site</td>
</tr>
</table>

</body>
</html>

Results:

Navigate
Page1
Page2
Page3
Page4

 Body content of your web site


See also:
What is PHP?
Tutorials