class User
{
    public $id;
    public $email;
    public $password;
    public $errors;

    public function isValid()
    {
        if (strpos($this->email, '@') === false) {
            $this->errors['email'] = 'Please enter an email address';
        }
        if (!$this->password) {
            $this->errors['password'] = 'Please enter a password';
        } elseif (strlen($this->password) < 4) {
            $this->errors['password'] = 'Please enter a longer password';
        }

        return !$this->errors;
    }
}

class UserDAO
{
    protected $conn;
    protected $logger;

    public function __construct(PDO $dbh, Logger $logger)
    {
        $this->dbh = $dbh;
        $this->logger = $logger;
    }

    public function getUsers()
    {
        $rows = null;
        try {
            $rows = $this->dbh->query("SELECT * FROM users")->fetchAll();
        } catch (PDOException $e) {
            $this->logger->log($e->getMessage(), __METHOD__);
        }
        return $rows;
    }

    public function getUserById($id)
    {
        $row = null;
        try {
            $sth = $this->dbh->prepare("SELECT * FROM users WHERE id = ?");
            $sth->execute(array($id));
            $row = $sth->fetchObject('User');
        } catch (PDOException $e) {
            $this->logger->log($e->getMessage(), __METHOD__);
        }
        return $row;
    }

    public function addUser(User &$user)
    {
        $success = false;
        try {
            $sth = $this->dbh->prepare("
                INSERT INTO users (email, password) VALUES (?, ?)
            ");
            $sth->execute(array($user->email, $user->password));
            if ($success = (bool) $sth->rowCount()) {
                $user->id = $this->dbh->lastInsertId();
            }
        } catch (PDOException $e) {
            $this->logger->log($e->getMessage(), __METHOD__);
        }
        return $success;
    }

    public function updateUser(User $user)
    {
        // ...
    }

    public function deleteUser($id)
    {
        // ...
    }

    public function isEmailUnique($email)
    {
        $count = 0;
        try {
            $sth = $this->dbh->prepare("SELECT COUNT(id) FROM users WHERE email = LOWER(?)");
            $sth->execute(array($email));
            $count = $sth->fetchColumn();
        } catch (PDOException $e) {
            $this->logger->log($e->getMessage(), __METHOD__);
        }
        return !$count;
    }
}

// ...

$userDAO = new UserDAO($dbh, $logger);
$user = new User();
$user->email = filter_input(INPUT_POST, 'email', FILTER_VALIDATE_EMAIL);
$user->password = filter_input(INPUT_POST, 'password');
// validate user
if ($user->isValid()) {
    // check if email address is unique (SO UGLY!)
    if ($userDAO->isEmailUnique($user->email)) {
        $user->errors['email'] = 'Please use a different email address';
    }
    // save user
    if ($user->addUser($user)) {
        // ...
    } else {
        // ...
    }
} else {
    // do something with $user->errors
}