WP AJAX is not working, always returns 0

The problem is where you place the add_action of your wp_ajax_*

Try to put it at the start of everything, and ensuring that it is being loaded in a /wp-admin/ route.

Try your url: http://xxx/wp-admin/admin-ajax.php?action=your_action until it gives you something different than zero.

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