PHP require file from root

I tried various other threads on this topic, but it did not work. So I write my specific case. I repeat, I have already tried other threads but trying what others advised I did not get results.

So, I'm creating my own little cms, are not very experienced. My structure is this:

/includes/config.php (class config.php) 
/index.php 
/other.php

Now I want to create an admin area, in a folder:

**/admin**

I would then call the config.php file in order to build queries and everything else, but I can not. I've tried this:

require_once $_SERVER['DOCUMENT_ROOT'] . '/includes/config.php';

But it does not work, how can I fix?

EDIT:

includes/config.php

class db {

private $conn;
private $host;
private $user;
private $password;
private $baseName;
private $port;
private $Debug;
    private $stmt;

    function __construct($params=array()) {
    $this->conn = false;
    $this->host = 'localhost'; //hostname
    $this->user = ''; //username
    $this->password = ''; //password
    $this->baseName = ''; //name of your database
    $this->port = '3306';
    $this->debug = true;
    $this->connect();
}

function __destruct() {
    $this->disconnect();
}

function connect() {
    if (!$this->conn) {
        try {
            $this->conn = new PDO('mysql:host='.$this->host.';dbname='.$this->baseName.'', $this->user, $this->password, array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'));  
        }
        catch (Exception $e) {
            die('Error : ' . $e->getMessage());
        }

        if (!$this->conn) {
            $this->status_fatal = true;
            echo 'Connection DB failed';
            die();
        } 
        else {
            $this->status_fatal = false;
        }
    }

    return $this->conn;
}

function disconnect() {
    if ($this->conn) {
        $this->conn = null;
    }
}


    public function query($query){
        $this->stmt = $this->conn->prepare($query);
    }


    public function bind($param, $value, $type = null){
            if (is_null($type)) {
                    switch (true) {
                        case is_int($value):
                            $type = PDO::PARAM_INT;
                            break;
                        case is_bool($value):
                            $type = PDO::PARAM_BOOL;
                            break;
                        case is_null($value):
                            $type = PDO::PARAM_NULL;
                            break;
                        default:
                            $type = PDO::PARAM_STR;
                    }
            }
        $this->stmt->bindValue($param, $value, $type);
    }

    public function execute(){
            return $this->stmt->execute();
    }        

    public function resultset(){
            $this->execute();
        return $this->stmt->fetchAll(PDO::FETCH_ASSOC);
    }        

    public function single(){
            $this->execute();
        return $this->stmt->fetch(PDO::FETCH_ASSOC);
    }

    public function rowCount(){
        return $this->stmt->rowCount();
    }

    public function lastInsertId(){
       return $this->conn->lastInsertId();
    }

    public function beginTransaction(){
        return $this->conn->beginTransaction();
    }

    public function endTransaction(){
        return $this->conn->commit();
    }

    public function cancelTransaction(){
        return $this->conn->rollBack();
    }

    public function debugDumpParams(){
        return $this->conn->debugDumpParams();
    }

}

$database = new db(); $pdo =& $database;