Lets say you have the following links:
<a href="index.php">Home</a><br>
<a href="about.php">About Us</a><br>
<a href="contact.php">Contact Us</a><br>
Copy and paste those into a file and call it nav.php and save it in the same directory as index, about, and contact.php are
Then, where you would want it to appear on your page, use
<? include("nav.php"); ?>
Now if you modify nav.php, it will update the navigation on all the pages.
Cgraz