Example of three layered Service

Author.php

<?php
class Author extends Entity {
   public $name;
   public $penNames;
}

AuthorDao.php

<?php
class AuthorDao extends Dao {
    public function getBooks($id) {
       $stmt = $this->db->prepare('...');
       $stmt->execute(array($id));
       return SomeMapper::mapResultToBooks($stmt);
    }

    public function updateAuthor(Author $author) {
      $stmt = $this->db->prepare('..');
      $stmt->execute(array($author->name, $author->penNames));
    }
}

AuthorService.php

<?php
interface AuthorService {
  public function getBooksByAuthor($id);
  public function updateAuthor(Author $author);
}

class AuthorServiceDaoImpl implements AuthorService {
  private $authorDao;
  public function getAuthorDao(AuthorDao $dao) {
    $this->authorDao = $authorDao;
  }

  public function getBooks($id) {
      return $this->authorDao->getBooks($id);
  }

  public function updateAuthor(Author $author) {
    try {
      $this->authorDao->getDB()->startTransaction();
      $this->authorDao->updateAuthor($author);
      $this->authorDao->commit();
    } catch(Exception $ex) {
       $this->authorDao->getDB()->rollback();
       throw $ex;
    }
  }
}

ControllerExample.php

<?php

//omitting setting exception mode, don't forget it!
$db = new PDO(...);
$authorDao = new AuthorDao($db);
$authorService = new AuthorServiceDaoImpl($authorDao);

$authorService->getBooks($author->getPrimaryKey());

//tip: you can simplify construction by using a Builder pattern, or a Factory pattern if you want to handle more than one implementation of the service.

Dao.php

<?php
class Dao {
   protected $db;
   public function __construct(PDO $db) {
     $this->db = $db;
   }

   public function getDB() {
      return $this->db;
   }
}

Entity.php

<?php
class Entity {
  private $primaryKey;
  public getPrimaryKey() {
    return $this->primaryKey;
  }

  public setPrimaryKey($primaryKey) {
    $this->primaryKey = $primaryKey;
  }
}