Here’s a reeeeaaaaally simple demo:
This is your index.php page:
Hi, I’m content
And I’m a footer
And in a file located in a folder called "includes", your navigation.html file:
It’s as simple as replacing everywhere your navigation is with this line:
<?php include(‘includes/navigation.html’); ?>
Then you only have to update one file (navigation.html) whenever you want to make a change to it.