Slim Application Error

The application could not run because of the following error:

Details

Type: ErrorException
Code: 2
Message: Undefined variable $DB_USERNAME
File: /var/www/html/api/db.php
Line: 6

Trace

#0 /var/www/html/api/db.php(6): Slim\Slim::handleErrors()
#1 /var/www/html/api/index.php(19): getDB()
#2 [internal function]: getUsers()
#3 /var/www/html/api/Slim/Route.php(462): call_user_func_array()
#4 /var/www/html/api/Slim/Slim.php(1326): Slim\Route->dispatch()
#5 /var/www/html/api/Slim/Middleware/Flash.php(85): Slim\Slim->call()
#6 /var/www/html/api/Slim/Middleware/MethodOverride.php(92): Slim\Middleware\Flash->call()
#7 /var/www/html/api/Slim/Middleware/PrettyExceptions.php(67): Slim\Middleware\MethodOverride->call()
#8 /var/www/html/api/Slim/Slim.php(1271): Slim\Middleware\PrettyExceptions->call()
#9 /var/www/html/api/index.php(14): Slim\Slim->run()
#10 {main}