Вы можете использовать стандартную функцию _user_mail_notify () из модуля "Пользователь" ядра Drupal.
// Create user.
$new_user = array(
'name' => $username,
'pass' => $password,
'mail' => $email,
'status' => 1,
'init' => $email,
'roles' => array(DRUPAL_AUTHENTICATED_RID => TRUE),
);
$account = user_save(NULL, $new_user);
// Set operation.
$op = 'register_no_approval_required';
// Send an email.
_user_mail_notify($op, $account);
Существуют различные значения $ op:
/* @param $op
* The operation being performed on the account. Possible values:
* - 'register_admin_created': Welcome message for user created by the admin.
* - 'register_no_approval_required': Welcome message when user
* self-registers.
* - 'register_pending_approval': Welcome message, user pending admin
* approval.
* - 'password_reset': Password recovery request.
* - 'status_activated': Account activated.
* - 'status_blocked': Account blocked.
* - 'cancel_confirm': Account cancellation request.
* - 'status_canceled': Account canceled.*/