These are operating system file permissions and have no bearing on the web server behavior. To deny access to a file being viewed directly from the browser (via an HTTP request) use the .htaccess file.
<Files config.inc.php>
order deny,allow
deny from all
</Files>
File permissions determine whether or not the web server (who is treated as a user) can access the file at all.
600 means only the owner of the file can read (view) or write the file. This will only work if PHP is being run with some type of su_exex.
Ordinarily a script (.php) is run as the user that called the script; in our case it would be the user the Apache (or other web server) is running as. That means that with 600 permissions, the file is owned by your user (FTP or CPanel user) and the web server can't read the file. In this situation, permissions need to be 644. The last number of the three indicates "other", or a user other than the owner (the second number indicates "group").
If the server is configured to run .php through some kind of su_exec (Switch User execute) then the user is switched to the owner of the .php file instead of the web server user. In that case 600 would be great, as the owner can read/write the file but nobody else can even see it.