WordPress – Include_once root files

I’m trying to include some .php files in the header.php of my WordPress Site.

Here is my schema:

Root/
    |_ includes
        |_ db_connect.php
        |_ functions.php
        |_ var_list.php
    |_ wp-includes
    |_ wp-content
        |_ themes
            |_ NameTheme
                |_ header.php
    |_ wp-admin

I need to include this lines on my header.php

include_once 'includes/db_connect.php';
include_once 'includes/functions.php';
include_once 'includes/var_list.php';

sec_session_start(); // function to start session, placed in functions.php

What I tryed:

1. With $_SERVER['DOCUMENT_ROOT']
2. With dirname(_FILE_)
3. With ABSPATH // this are define on my wp-config.php
4. With home_url
5. With site_url

All return blank page (500 internal error)
Can help me, please?
Thanks.