Fatal error: Call to undefined function register_new_user()

I greped a 3.6 install and got this:

./wp-login.php:321:function register_new_user( $user_login, $user_email ) {
./wp-login.php:550:             $errors = register_new_user($user_login, $user_email);

Notice the function definition is in wp-login.php.

If you look at the current wp-login.php, it isn’t there anymore and there is a note at the top of the page, just above the code:

Last change on this file was 25231, checked in by SergeyBiryukov, 11
days ago

Move check_password_reset_key(), reset_password(), and
register_new_user() from wp-login.php to wp-includes/user.php, to make
them reusable. props beaulebens for initial patch. fixes #20279.

That migration of code has been needed for a long, long time. I am glad it was finally done. Your fix is to update WordPress.