Whilsts @Shaon’s answer is perfectly valid, I’m of the opinion you really shouldn’t be throwing configuration settings around as global variables – use a constant instead.
/* Constants are in the global scope, but can only be defined once. */
define( 'FEEDBURNER_USERNAME', 'my_username' );
define( 'FEEDBURNER_PASSWORD', 'my_password' );
If the variable needs to be dynamic (for example, if it’s an option in the database), use a function that returns it:
function get_feedburner_config( $option = null )
{
$config = get_option( 'feedburner' );
$config = wp_parse_args( $config, array(
'username' => 'default',
'password' => 'default'
));
if ( $option )
return isset( $config[ $option ] ) ? $config[ $option ] : '';
return $config;
}