[Xenon]

"is_writable("./config.php")"

this will just be true, if file config.php already exists and php can write into it, but you are in the part of the file_exists("./config.php") - if clause where the file surely isn't created, so this condition can never be true

if you replace is_writable("./config.php") with a true, you will see that it'll work then (or at least gives you a no permission error if php has no permissions to create files)