如何设计像Stack Overflow这样的网站，顶部有一个常量菜单，只有一个刷新的主体？

How would I go about designing a site with a similar layout to Stack Overflow (top menu bar that is always visible) and content body that reloads. I tried using ajax .load() to load in content to the main area but then I have issues with JavaScript not working. How does Stack Overflow accomplish this? I would like to do this in PHP.How would I go about designing a site with a si