register_new_user( string $user_login, string $user_email )

function register_new_user( $user_login, $user_email ) {
	$errors = new WP_Error();

	$sanitized_user_login = sanitize_user( $user_login );
	/**
	 * Filters the email address of a user being registered.
	 *
	 * @since WP-2.1.0
	 *
	 * @param string $user_email The email address of the new user.
	 */
	$user_email = apply_filters( 'user_registration_email', $user_email );

	// Check the username
	if ( $sanitized_user_login == '' ) {
		$errors->add( 'empty_username', __( '<strong>ERROR</strong>: Please enter a username.' ) );
	} elseif ( ! validate_username( $user_login ) ) {
		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username.' ) );
		$sanitized_user_login = '';
	} elseif ( username_exists( $sanitized_user_login ) ) {
		$errors->add( 'username_exists', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ) );

	} else {
		/** This filter is documented in wp-includes/user.php */
		$illegal_user_logins = array_map( 'strtolower', (array) apply_filters( 'illegal_user_logins', array() ) );
		if ( in_array( strtolower( $sanitized_user_login ), $illegal_user_logins ) ) {
			$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: Sorry, that username is not allowed.' ) );
		}
	}

	// Check the email address
	if ( $user_email == '' ) {
		$errors->add( 'empty_email', __( '<strong>ERROR</strong>: Please type your email address.' ) );
	} elseif ( ! is_email( $user_email ) ) {
		$errors->add( 'invalid_email', __( '<strong>ERROR</strong>: The email address isn&#8217;t correct.' ) );
		$user_email = '';
	} elseif ( email_exists( $user_email ) ) {
		$errors->add( 'email_exists', __( '<strong>ERROR</strong>: This email is already registered, please choose another one.' ) );
	}

	/**
	 * Fires when submitting registration form data, before the user is created.
	 *
	 * @since WP-2.1.0
	 *
	 * @param string   $sanitized_user_login The submitted username after being sanitized.
	 * @param string   $user_email           The submitted email.
	 * @param WP_Error $errors               Contains any errors with submitted username and email,
	 *                                       e.g., an empty field, an invalid username or email,
	 *                                       or an existing username or email.
	 */
	do_action( 'register_post', $sanitized_user_login, $user_email, $errors );

	/**
	 * Filters the errors encountered when a new user is being registered.
	 *
	 * The filtered WP_Error object may, for example, contain errors for an invalid
	 * or existing username or email address. A WP_Error object should always returned,
	 * but may or may not contain errors.
	 *
	 * If any errors are present in $errors, this will abort the user's registration.
	 *
	 * @since WP-2.1.0
	 *
	 * @param WP_Error $errors               A WP_Error object containing any errors encountered
	 *                                       during registration.
	 * @param string   $sanitized_user_login User's username after it has been sanitized.
	 * @param string   $user_email           User's email.
	 */
	$errors = apply_filters( 'registration_errors', $errors, $sanitized_user_login, $user_email );

	if ( $errors->get_error_code() )
		return $errors;

	$user_pass = wp_generate_password( 12, false );
	$user_id = wp_create_user( $sanitized_user_login, $user_pass, $user_email );
	if ( ! $user_id || is_wp_error( $user_id ) ) {
		$errors->add( 'registerfail', sprintf( __( '<strong>ERROR</strong>: Couldn&#8217;t register you&hellip; please contact the <a href="mailto:%s">webmaster</a> !' ), get_option( 'admin_email' ) ) );
		return $errors;
	}

	update_user_option( $user_id, 'default_password_nag', true, true ); //Set up the Password change nag.

	/**
	 * Fires after a new user registration has been recorded.
	 *
	 * @since WP-4.4.0
	 *
	 * @param int $user_id ID of the newly registered user.
	 */
	do_action( 'register_new_user', $user_id );

	return $user_id;
}