How to hide that a certain non-WordPress directory exists on the server?

If you want to block direct access to the other folder, you can put a .htaccess file in that folder that contains just:

deny from all

That way you cannot open any file from that folder directly, but you can include them (in PHP) without any problems.

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