<?
  // ... ...
$conn->autocommit(FALSE);
$conn->begin_transaction();

  for ($i=0; $i < count($users); $i++){
    $user = $users[$i];
    $nickname = $nicknames[ mt_rand(0, count($nicknames)-1 )];
    $password = password_hash('123', PASSWORD_DEFAULT);

    $createUser = "INSERT INTO `users` (`id`, `username`, `nickname`, `password`) VALUES (NULL, '{$user}', '{$nickname}', '{$password}') ";
    if ($conn->query($createUser)) {
      echo "good! create a user: '$user'. <br> ";
    } else {
        echo " Error: {$conn->error} :
                        sql: {$createUser}  ";
    }
  }

$conn->commit();

?>