Path to includes are not resolving when define is used

in a Php document path to a class specified with a string resolved without a problem:

require_once ( './classes/class.php' );

However, if a defined variable is used, the path does not resolve and Dreamweaver doesn't help with method hints:

define ( CLASS_DIR . './classes/' );

require_once ( CLASS_DIR . 'class.php' );

As a note,  CLASS_DIR might not be defined in the same document, bu linked from another one that contains the define:

require_once ( './config.php' ); //contains  define ( CLASS_DIR . './classes/' );

As I use defined paths, I can take no advantage of Dreamweaver's hints.