<?php

use Drupal\user\Entity\User;

/**
 * Rehash migrated Drupal 5 MD5 hashes for Drupal 8.
 */
function example_update_8101(&$sandbox) {

  // This update hook should only be ran if the user migration didn't process
  // the MD5 passwords with the 'md5_passwords' configuration option. Ideally
  // the user migration should handle this re-hashing.

  /** @var \Drupal\Core\Password\PasswordInterface $password */
  $password = \Drupal::service('password');
  // We exclude user records already having the correct hash.
  $hashType = '$S$';

  if (!isset($sandbox['progress'])) {

    $user_count = \Drupal::entityQuery('user')
      ->condition('pass', $hashType . '%', 'NOT LIKE')
      ->count()
      ->execute();

    $sandbox['progress'] = 0;
    $sandbox['limit'] = 500;
    $sandbox['total'] = $user_count;
  }

  $uids = \Drupal::entityQuery('user')
    ->condition('pass', $hashType . '%', 'NOT LIKE')
    ->range($sandbox['progress'], $sandbox['progress'] + $sandbox['limit'])
    ->execute();

  foreach ($uids as $uid) {
    $user = User::load($uid);
    // @see \Drupal\user\Plugin\migrate\destination\EntityUser::save()
    // @see \Drupal\Core\Field\Plugin\Field\FieldType\PasswordItem::preSave()
    $user->pass->pre_hashed = TRUE;
    $user->pass->value = 'U' . $password->hash($user->pass->value);
    $user->save();
    $sandbox['progress']++;
  }

  $sandbox['#finished'] = ($sandbox['progress'] / $sandbox['total']);
}