if ($password === $repeatpass) {
mysqli_query($connect, "INSERT INTO `users` (`id`, `full_name`, `login`, `email`, `password`) VALUES (NULL, '$full_name', '$login', '$email', '$password')");
$_SESSION['message'] = 'Регистрация прошла успешно!';
header('Location: /index.php');
}
if (!mysqli_query($connect, "INSERT INTO `users` .......")) {
die (mysqli_error($connect));
}