Well, if your menu bar is plain HTML, you can save that HTML in a separate file (menu_bar.html might be a good name), and just include it in your PHP script:
However, you're really talking about page templates. A template is a basic layout for a page that specifies where everything goes, what colors, images, etc. belong on a page, but does NOT contain content.
There are many ways of using templates. I suggest starting with SMARTY to learn about PHP-driven templates.