Assign additional membership levels based on fields at checkout.

my_amlbofat_save_function.php

<?php
/**
 * Assign additional membership levels based on fields at checkout.
 * Requires PMPro, PMPro MMPU, and PMPro Register Helper.
 * This is just an example. Be sure to change the field settings,
 * field meta name, and membership levels to fit your needs.
 */

// Add a checkbox via Register Helper
function my_amlbofat_add_fields() {
	// Don't break if Register Helper is not loaded.
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return false;
	}

	// Define the fields.
	$fields = array();
	$fields[] = new PMProRH_Field(
		'include_level_2',
		'checkbox',
		array(
			'label'		=> 'Include Level 2 Content?',
			'save_function'  => 'my_amlbofat_save_function',	// !! NOTE !!
			'profile'   => true,
		)
	);

	// Add the fields into a new checkout_boxes are of the checkout page.
	foreach ( $fields as $field ) {
		pmprorh_add_registration_field( 'checkout_boxes', $field );
	}
}
add_action( 'init', 'my_amlbofat_add_fields' );

// Give user's an extra level based on a Register Helper field.
function my_amlbofat_save_function( $user_id, $field_name, $value ) {	
	// Make sure MMPU is active.
	if ( ! defined( 'PMPROMMPU_VER' ) ) {
		return;
	}

	// Check field and give user level if appropriate.
	if ( $field_name == 'include_level_2' ) {
		if ( $value == 1 ) {
			pmprommpu_addMembershipLevel( 2, $user_id );
			update_user_meta( $user_id, 'include_level_2', 1 );
		} else {
			pmpro_cancelMembershipLevel( 2, $user_id );
			update_user_meta( $user_id, 'include_level_2', 0 );
		}
	}
}

This recipe is included in the blog post on "Assign Additional Membership Levels Based on Fields at Checkout" at Paid Memberships Pro here: https://www.paidmembershipspro.com/assign-additional-membership-levels-based-on-fields-at-checkout/