similar to these questions:
define('_DB_USER_', 'something else');
define('_DB_PASSWD_', 'and another value');
sed "s/\(define('_DB_NAME_', \).*/\1'new value');/" filename
and likewise for the other two.
If you want to alter the file in place, you can use
-i, but you have to be careful, different versions of sed handle that differently.