通过外部脚本调用函数codeigniter脚本

My situation is:

• I have a project with codeigniter working
• Inside of controllers i need to call a function of a php script
• I have other script running background and when it get data, send to codeigniter controller

code:

first line:

include('/opt/lampp/htdocs/myproject/application/controllers/myFile.php');

This script is running backgroud.

function procmsg($topic,$msg){
    //do something...
    //my_function() is a function in the myFile.php script
    my_function(true, $userGtwy, $passwdGtwy, $node_id, $idGateway, $value, $rssi);
}

myFile.php

First lines of this script:

require(APPPATH.'/libraries/REST_Controller.php');
if ( ! defined('BASEPATH')) exit('No direct script access allowed');

The function:

public function my_function($flag=false, $usr, $pass, $node, $gateway, $val, $freq){}

When I run my first script i get:

PHP Notice:  Use of undefined constant APPPATH - assumed 'APPPATH' in /opt/lampp/htdocs/myproject/application/controllers/myFile.php on line 3
PHP Warning:  require(APPPATH/libraries/REST_Controller.php): failed to open stream: No such file or directory in /opt/lampp/htdocs/myproject/application/controllers/myFile.php on line 3
PHP Fatal error:  require(): Failed opening required 'APPPATH/libraries/REST_Controller.php' (include_path='.:/usr/share/php') in /opt/lampp/htdocs/myproject/application/controllers/myFile.php on line 3

and, if i comment this line:

require(APPPATH.'/libraries/REST_Controller.php');

i get: No direct script access allowed I need this library and I need access with my script, help me please!

I found this: Access Codeigniter functions from external script But i don't understand...

My question, can i do that? or is it impossible?