First, I would suggest creating a menu class to handle the creation of this menu and call the class method in your index.php. This would separate the code from the presentation, provide abstraction, etc. When you call it you could pass it the page, and the class takes care of the implementation and passes the complete menu back.
Second, I would suggest using a switch statement instead of if...else with lots of or conditions. You can cascade cases by not including the break keyword:
switch( $page) {
case: 'home':
$current_home = 'current';
break;
case: 'users.online':
case: 'users.location':
case: 'users.featured':
.......
$current_users = 'current';
break;
}
As far as your PHP use goes, you're not using much. As I said with your if statements, I think a switch would do much better, the $page would be evaluated once and matched once, unlike each if. If you could use an loop for the menu, it could cut down on the amount of actual literal HTML in your code. If you created arrays to hold each main menu piece (maybe what the others meant), then you could loop over the array and combine the array content, some html, and your variables to create the menu system. Less literal HTML and more efficient use of the PHP.