Since a few days there is some change for classes using a MySQL database:
In the past the the constant variables are located in the db_config file which is located (by default) in the folder where the class file is also located.
Because many users (including me) are using 2 or more classes in one project it's needed to use only one file with the database connection variables. The file db.php contains this vars now and is located (by default) in the folder classes (one above the root).