Datamapper pattern in PHP

abstract class MapperAbstract
{
    /**
      * @var Zend_Db_Adapter_Pdo_Mysql
     */
    private $dbAdapter;
 
    /**
     * Create a new instance of the domain object. If the $data parameter is specified
     * then the object will be populated with it.
     * @param array $data
     * @return DomainObjectAbstract
     */
    public function create(array $data = null)
    {
        $obj = $this->_create();
        if ($data)
        {
            $obj = $this->populate($obj, $data);
        }
        return $obj;
    }
 
    /**
     * Save the domain object
     *
     * @param DomainObjectAbstract $obj
     */
    public function save(Application_Model_Abstract $obj)
    {
        if (is_null($obj->getId()))
        {
            $this->_insert($obj);
	}
	else
	{
            $this->_update($obj);
        }
    }
 
    /**
     * Delete the domain object
     * @param DomainObjectAbstract $obj
     */
    public function delete(Application_Model_Abstract $obj)
    {
        $this->_delete($obj);
    }
 
    /**
     * Populate the domain object with the values from the data array.
     *
     * @param DomainObjectAbstract $obj
     * @param array $data
     * @return DomainObjectAbstract
     */
    abstract public function populate(Application_Model_Abstract $obj, array $data);
 
    /**
     * Create a new instance of a domain object
     *
     * @return DomainObjectAbstract
     */
    abstract protected function _create();
 
    /**
     * Insert the domain object into the database
     *
     * @param DomainObjectAbstract $obj
     */
    abstract protected function _insert(Application_Model_Abstract $obj);
 
    /**
     * Update the domain object in persistent storage
     *
     * @param DomainObjectAbstract $obj
     */
    abstract protected function _update(Application_Model_Abstract $obj);
 
    /**
     * Delete the domain object from peristent Storage
     *
     * @param DomainObjectAbstract $obj
     */
    abstract protected function _delete(Application_Model_Abstract $obj);
}

class UserMapper extends MapperAbstract
{
	private $dbAdapter;
 
	public function __construct($dbAdapter)
	{
		$this->dbAdapter = $dbAdapter;
	}
 
	/**
	* Create a new User domainobject
	*/
	protected function _create()
	{
		return new User();
	}
 
	/**
    * Insert the DomainObject in persistent storage
    *
    * @param DomainObjectAbstract $obj
	*/
	protected function _insert(DomainAbstract $obj)
	{
		// Insert User object into database
		/* @var $obj Application_Model_User */
		$query = 'INSERT INTO users (username,password) VALUES (?,SHA1(?))';
		$ret = $this->dbAdapter->query($query, array($obj->getUsername(),
								$obj->getPassword())
                );
 
		// Assign the new user id to the User object
		$obj->setId($this->dbAdapter->lastInsertId());
	}
 
	/**
	* Update
	*
	* @param DomainObjectAbstract $obj
	*/
	protected function _update(DomainAbstract $obj)
	{
		// Update User object in database
			$query = 'UPDATE users SET username=?,password=SHA1(?) WHERE id=?';
			$ret = $this->dbAdapter->query($query, array($obj->getUsername(),
									$obj->getPassword(),
									$obj->getId()
							));
	}
 
	/**
	* Delete the User from the database
	*
	* @param DomainObjectAbstract $obj
	*/
	protected function _delete(DomainAbstract $obj)
	{
		$query = 'DELETE FROM users WHERE id=?';
		$ret = $this->dbAdapter->query($query, array($obj->getId()));
    }
 
	/**
	* Populate the User (DomainObject) with the data array.
	*
	* @param DomainObjectAbstract $obj
	* @param array $data
	* @return User
	*/
	public function populate(DomainAbstract $obj, array $data)
	{
		if (isset($data['id']))
		{
			$obj->setId($data['id']);
		}
 
		if (isset($data['username']))
		{
			$obj->setUsername($data['username']);
		}
 
		if (isset($data['password']))
		{
			$obj->setPassword($data['password']);
		}
 
		return $obj;
	}
 
 
	/**
	 * Retrieves user by userid
	 * 
	 * @param int $userId
	 * @return User (null if not found)
	 */
	public function findById($userId)
	{
		$sql = "SELECT id,username,password FROM users WHERE id=?";
 
		$data = $this->dbAdapter->fetchRow($sql, array($userId), Zend_Db::FETCH_ASSOC);
 
		$user = null;
 
		if ($data != false)
		{
			$user = $this->create($data);
		}
 
		return $user;
	}
 
	/**
	 * Retrieves user by username
	 * 
	 * @param string $username
	 * @return User (null if not found)
	 */
	public function findByUsername($username)
	{
		$sql = "SELECT id,username,password FROM users WHERE username=?";
 
		$data = $this->dbAdapter->fetchRow($sql, array($username), Zend_Db::FETCH_ASSOC);
 
		$user = null;
 
		if ($data != false)
		{
			$user = $this->create();
		}
 
		return $user;
	}
}

$userMapper = new UserMapper($dbAdapter);
$user = $userMapper->findByUsername('ruben');

$userMapper = new UserMapper($dbAdapter);
$user = $userMapper->findByUsername('ruben');
$user->setPassword('newpassword');
$userMapper->save($user);