First steps with PHP OOP

I am learning OOP using PHP5 and I wrote two simple classes for receive data of database using dependency injection pattern and render a grid with Twig. Please, if is possible, take a look and give me any feedback.

My question is: does this code have sense for OOP logic?

Sorry About my English

index.php

<?php   
/*
 * Replace with autoload
 */

require_once 'class/class.db.php';
require_once 'class/class.entries.php';
require_once 'vendor/autoload.php'; 
Twig_Autoloader::register();


$_db = new Db('localhost', 'root', '', 'tgsm_ascm');
$entries = new Entries( $_db );
$arEntries = $entries->getLastEntries();


$loader = new Twig_Loader_Filesystem('tpl/');
$twig = new Twig_Environment($loader);

$template = $twig->loadTemplate('ctbaGrillaActividades.twig');

echo $template->render( array( 'arEntries' => $arEntries ) );
?>

class.db.php

<?php
/*
 * Database manager
 * @autor Rebolini Pablo Demian <[email protected]>
 */
class Db
{
    protected $db = NULL;

    private $dbServer,
            $dbUser,
            $dbPassword,
            $dbName;

    /*
     * Constuct
     * @args string $dbSrv
     * @args string $dbUsr
     * @args string $dbPsw
     * @args string $dbName
     * @return null
     */
    public function __construct( $dbSrv, $dbUsr, $dbPsw, $dbName  )
    {
        $this->dbServer     = $dbSrv;
        $this->dbUser   = $dbUsr;
        $this->dbName       = $dbName;
        $this->dbPassword       = $dbPsw;

        if( is_null( $this->db ) )
        {
            $this->conect(  $this->dbServer, $this->dbUser, $this->dbPassword, $this->dbName );
        }
    }


    /*
     * Conecta a la BD
     * @args string $dbServidor
     * @args string $dbUsuario
     * @args string $dbClave
     * @args string $dbNombre
     * @return null
     */
    public function conect( $dbSrv="localhost", $dbUsr="root", $dbPws="", $dbName="tgsm_ascm" )
    {
        $dsn = "mysql:dbname={$dbName};host={$dbSrv}";

        try
        {
            $this->db = new PDO( $dsn, $dbUsr, $dbPws );
        }
        catch ( PDOException $e )
        {
            echo 'Falló la conexión: ' . $e->getMessage();
        }       
    }

    /*
     * Desconecta de la DB
     * @return null
     */
    public function disconect( )
    {
        $this->db = NULL;
    }

    /*
     * Execute Direct SQL Queries
     * @args string $query
     * @args string $fetchOrFetchAll
     * @args string $placeHolder
     * @args string $retunrDataType
     */ 
    public function directQuery( $query, $fetchOrFetchAll='fetch', $placeHolder='', $returnDataType=PDO::FETCH_ASSOC )
    {
        if( is_array( $placeHolder ) )
        {
            $p = $this->db->prepare( $query );  
            $p->setFetchMode( $returnDataType );

            if( $p->execute($placeHolder) ) {
                return ($fetchOrFetchAll=='fetch') ? $p->fetch() : $p->fetchAll();
            } else {
                return false;
            }
        }
        else
        {
            $p = $this->db->prepare( $query );   
            $p->setFetchMode( $returnDataType );
            if( $p->execute() ) {
                return ($fetchOrFetchAll=='fetch') ? $p->fetch() : $p->fetchAll();
            } else {
                return false;
            }
        }
    }
}
?>

class.entradas.php

<?php
/* 
 * Clase Maestra de manejo de entradas
 * @autor Rebolini Pablo Demian <[email protected]>
 */
class Entries
{
    private $db = NULL;
    public function __construct(Db $db)
    {
        $this->db = $db;
    }



    public function getLastEntries()
    {
        $_sql = "SELECT ascm_entradas.ID,
                        ascm_entradas.autor,
                        ascm_entradas.titulo,
                        ascm_entradas.leyenda,
                        ascm_entradas.img_miniatura
                FROM
                        ascm_entradas
                WHERE
                        ascm_entradas.`status` =  '2'
                ORDER BY
                        ascm_entradas.ID DESC
                LIMIT 0, 15";

        return $this->db->directQuery( $_sql, 'fetchAll', '' );
    }


    /*
     * Debug
     * @return string
     */
    public function debug()
    {
        return print_r( $this->db );
    }

Thank you