Custom user registration process

gistfile1.php

<?php

/**
 * Process user registration
 *
 * A custom user registration process that listens for a
 * specific type POST request that will create a new user
 * on the blog.
 *
 * @action wp
 *
 * @return void
 */
function fjarrett_user_registration() {
	if (
		'POST' !== $_SERVER['REQUEST_METHOD']
		||
		! isset( $_POST['registration_token'] )
		||
		! isset( $_POST['registration_nonce'] )
		||
		false === wp_verify_nonce( $_POST['registration_nonce'], 'user_registration-' . $_POST['registration_token'] )
		||
		! isset( $_POST['user_login'] )
		||
		! isset( $_POST['user_pass'] )
		||
		! isset( $_POST['first_name'] )
		||
		! isset( $_POST['last_name'] )
		||
		! isset( $_POST['user_email'] )
	) {
		return;
	}

	if ( is_user_logged_in() ) {
		wp_die( 'Sorry, you cannot register because you are already logged in.' );
	}

	$user_login = sanitize_user( filter_input( INPUT_POST, 'user_login' ) );
	$user_pass  = filter_input( INPUT_POST, 'user_pass' ); // Unsanitized
	$first_name = sanitize_text_field( filter_input( INPUT_POST, 'first_name' ) );
	$last_name  = sanitize_text_field( filter_input( INPUT_POST, 'last_name' ) );
	$user_email = sanitize_email( filter_input( INPUT_POST, 'user_email' ) );

	if (
		empty( $user_login )
		||
		empty( $first_name )
		||
		empty( $last_name )
		||
		empty( $user_email )
		||
		empty( $user_pass )
	) {
		wp_die( 'Some required fields were missing, please try again.' );
	}

	$user_id = wp_create_user( $user_login, $user_pass, $user_email );

	if ( is_wp_error( $user_id ) ) {
		wp_die( $user_id->get_error_message() );
	}

	$user_args = array(
		'ID'           => $user_id,
		'first_name'   => $first_name,
		'last_name'    => $last_name,
		'display_name' => trim( sprintf( '%s %s', $first_name, $last_name ) ),
	);

	$user_id = wp_update_user( $user_args );

	if ( is_wp_error( $user_id ) ) {
		wp_die( $user_id->get_error_message() );
	}

	$creds = array(
		'user_login'    => $user_login,
		'user_password' => $user_pass,
		'remember'      => true,
	);

	$user = wp_signon( $creds, false );

	if ( is_wp_error( $user ) ) {
		wp_die( $user->get_error_message() );
	}

	wp_redirect( home_url() );

	exit;
}
add_action( 'wp', 'fjarrett_user_registration' );

See registration form here https://gist.github.com/fjarrett/1c122226e56e6f352b4e