If you look at the PHP constant PATH_SEPARATOR, you will see it being ":" for you.
If you break apart your string ".:/usr/share/pear:/usr/share/php" using that character, you will get 3 parts.
Any attempts to include()/require() things, will look in these directories, in this order.
It is showing you that in the error message to let you know where it could NOT find the file you were trying to require()
For your first require, if that is being included from your index.php, then you dont need the dir stuff, just do...
require_once ( 'db/config.php');