Well - you will need to learn how to do this stuff and here's where you can learn to do it.
W3Schools Online Web Tutorials[
^]
To get you going: a php file will be accessed by
AJAX to get your new content to replace 'hello'. You will need
javaScript to access
DOM in order to update a particular location on the page without redrawing the page.
You could also use php to draw the original page and then to redraw it with new values - but that's not going to get you far and is very cumbersome,