Example function return run through a WordPress filter.

wp-function-filter-return.php

<?php
/**
 * Gets the username of the logicmelon feed as set on the plugins
 * settings page in the admin.
 *
 * @return string the username of the feed or empty string if not set.
 */
function hdji_get_username() {
	return apply_filters( 'hdji_get_username', get_option( 'hdji_username' ) );
}