//Index.php
<?php
define("current_page","home");
include "nav.php";
?>

//Diger.php
<?php
define("current_page","diger");
include "nav.php";
?>

//Nav.php
<?php
switch(current_page){
case "home":
echo "anasayfa";
break;
case "diger":
echo "diğer sayfa";
break;
}
?>