you can't change how PHP is coded via javascript. You can send variables to php via javascript and then PHP responds to those variables... it looks like you have a registration of sorts. If you're using standard HTTP requests, you could use javascript to append a $_GET variable to the action link. For Instance:
$('#form').attr('action', $('#form').attr('action') + '?page=welcome');
Then, upon clicking the link, PHP will have access to $_GET['page'], so in php you could:
switch($_GET['page'])) {
case 'welcome':
include 'section/welcome.html';
break;
case 'nda':
include 'section/nda.html';
break;
}