I am trying to figure out how to load contents without refreshing navigation bar like facebook does.
If you look into a facebook profile (not timeline), there is a navigation bar in the left column which has 'wall', 'info', 'photos', 'friends', etc..
And for example, when you click 'wall', the url becomes
PHP is server side, and thus if you want to refresh one bit of the page, the whole page has to be sent back to the server since that's where it will decide what part to refresh.
What you could do is use iframes. An iframe with the menu, an iframe with the content, then you can only refresh the content iframe. Though, I'm not really a fan of frames for various reasons. Using Jscript would be better :)
Since a couple of years, Single Page Applications (webapps that run entirely in the browser, without having to do a full refresh on the server) have increased a lot in popularity.