PhP link include page

How can i make a link include a page inside my index page, without reloading the whole website.
I thought of this:
<a href="<?php $page=1 ?>">Page</a> //this link is in index.php

<?php if($page=1){
include_once('example.php');
}
?>

It doesn't work, I know, but is there a solution to this!?