function get_user_id arrowchat

I try to install Arrowchat on my website but don't find userid

the original cod from Arrowchat is this

/**
     * This function returns the user ID of the logged in user on your site.  Technical support will not
     * help you with this for stand-alone installations.  You must purchase the professional installation
     * if you are having trouble.
     *
     * Suggestion: Check out the other integration files in the functions/integrations directory for
     * many examples of how this can be done.  The easiest way is to get the user ID through a cookie.
     *
     * @return the user ID of the logged in user or NULL if not logged in
     */

    function get_user_id() 
    {

        $userid = NULL;

        if (isset($_COOKIE['userid']))
        {
            $userid = $_COOKIE['userid'];
        }

        return $userid;

    }

This is in config.php

define('DB_USERTABLE','users'); 
define('DB_USERTABLE_NAME','username'); 
define('DB_USERTABLE_USERID','idu'); 
define('DB_USERTABLE_AVATAR','image');

My userid from mysql is idu