how to access config items

I tried this. It fails.

Trying to access items I have added to
application/config/config.php within a cronjob
Maybe this is just something that can't be done?

<?php
// include './lib/inc_dbconnect.php';
// Database Connections

// the below fails:

//$db_host     = $this->config->item('db_host');
//$db_database = $this->config->item('db_database');
//$db_username = $this->config->item('db_username');
//$db_password = $this->config->item('db_password');    

// the below fails also:

$db_host     = config_item('db_host');
$db_database = config_item('db_database');
$db_username = config_item('db_username');
$db_password = config_item('db_password');    
?>