How to add active state to the navigation links?

In your PHP file, set $thisPage before you include('nav.php');. Then, the variable $thisPage will be passed to nav.php and processed in its if statement.

This is really bad practice though, and a better way of doing it would be by creating your menu in WordPress itself and hooking it into your template. WordPress documents how to do this well.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)