class User extends stdClass {
    function getName() {
        return $this->first_name . " ". $this->last_name;
    }
}

class Post extends stdClass {

    /**
     * @var User
     */
    public $author;
}

DB::getInstance()->setFetchTableNames(1);
$sql = DB::getInstance()
    ->select("p.*, u.*")
    ->from("posts p")
    ->join("INNER JOIN users u USING(user_id)")
    ->where("u.user_id = ?", $user_id)
    ->orderBy("p.title");

$stmt = $sql->execute();

/* @var Post[] $post_collection  */
$post_collection = array();

while($post = $stmt->fetchInto(new Post, "p")) {
    $post->author = $stmt->fetchIntoFromLastRow(new User, "u");
    $post_collection[] = $post;
}

// Usage
foreach($post_collection as $post) {
    echo $post->author->getName();
}

class User {
    public
        $first_name,
        $last_name
    ;
    //etc...
}

$db = DB::getInstance();
$db->setFetchTableNames( 1 );
$sql = $db->select(//etc...

while( $post = $stmt->fetchInto( new Post(), 'p' ) ) {
    $post->author = $stmt->fetchIntoFromLastRow( new User(), 'u' );
    echo $post->author->getName();
}