Usually the way you'd accomplish this would be to put your menu in a separate file and function, and include that file where you need it. For example:
This is an effective way to split up your website into multiple pieces, which is very useful when you want to add a button to your menu for instance. Then you only have to change it in one place, and it's updated on every page of your website.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<p>something else goes here</p>