public function login($email, $password)
{
    $sql = 'SELECT * FROM user WHERE email = :email';
    $db = new Db();
    $result = $db->query($sql, self::class, [':email' => $email])[0];

    if (password_verify($password, $result->getPassword())) {
        //Как здесь записать в текущий объект все свойства объекта $result?
        return $this;
    } else {
        $error = 'Неправильная пара логинпароль';
        return $error;
    }
}

public static function login($email, $password): User
{
    $sql = 'SELECT * FROM user WHERE email = :email';
    $db = new Db(); // todo: extract concrete class usage
    $users = $db->query($sql, self::class, [':email' => $email]);

    if (!empty($users) && password_verify($password, $users[0]->getPassword())) {
        return $users[0];
    }
    throw new AuthException('Неправильная пара логинпароль');
}

class User implements JsonSerializable {
    private $username;
    private $password;
    private $name;

    private static function findByUsername($username) {
        $res = new self();
        $res->username = $username;
        $res->name = 'Some User';
        $res->password = password_hash("aXSA9_5uZp0QLhGY", PASSWORD_DEFAULT);
        return $res;
    }

    public function login($username, $password) {
        $user = self::findByUsername($username);
        if (password_verify($password, $user->password)) {
            $fields = get_class_vars(static::class);
            foreach ($fields as $field => $_) {
                $this->$field = $user->$field;
            }
        }
    }

    public function jsonSerialize() {
        return [
            'username' => $this->username,
            'name' => $this->name
        ];
    }
}

$user = new User();
$user->login('admin', 'aXSA9_5uZp0QLhGY');
echo json_encode($user); // {"username":"admin","name":"Some User"}