User

To make a redirection after user login, we can use the hook hook_user_login().
Note : You cannot use $form_state->setRedirectUrl() directly in the form alter, since it will be overwritten by UserForm::submitForm().

For this, You must use hook_entity_delete.

Example:
$user = User::load($uid);
user_login_finalize($user);

Get All roles:
    $roles = \Drupal\user\Entity\Role::loadMultiple();

Get a user's roles:
    $uid = 4; //The user ID
    $user = \Drupal\user\Entity\User::load($uid);
    $roles = $user->getRoles();

Example:

    //use Drupal\user\Entity\User;
    $userCurrent = \Drupal::currentUser();
    $user = \Drupal\user\Entity\User::load($userCurrent->id());
    $name = $user->getUsername();

Drupal do not like using the $_SERVER['REMOTE_ADDR']; to get the remote ip (the user ip address).

In Drupal 7, the function ip_address() exist but not in D8.
$ip = ip_address();

Store data on session.

Example:

$session = new \Symfony\Component\HttpFoundation\Session\Session();
$session->set('var_name', $session->get('var_name',0) + 1);
drupal_set_message($session->get('var_name'));

    Here an example for create a user account programmatically in Drupal 8

    $language = \Drupal::languageManager()->getCurrentLanguage()->getId();
    $user = \Drupal\user\Entity\User::create();

Drupal 8

//To get the lanuage code:
$language = \Drupal::languageManager()->getCurrentLanguage()->getId();

//To get the language name:
$language =  \Drupal::languageManager()->getCurrentLanguage()->getName();