I'm writing a wordpress module that will have to re-direct the registration process to some checkout. I've the checkout working, and that i have added the additional options I have to the standard Wordpress registration screen.

I want some pointers regarding how to intercept the registration process before anything is devoted to the database. Following the checkout phase I have to then re-start the proccess and finished it.

Take a look at wp-login.php - that consists of virtually all of the login and registration functions.

You will see online 300 (by 3.), you are able to hook to the action register_post, that is known as right before Wordpress tries to save the brand new user towards the DB

function my_register_post($sanitized_user_login, $user_email, $errors)
{
    // do your stuff!
}
add_action('register_post', 'my_register_post', 10, 3);