Send users' next payment date to Mailchimp.

mailchimp_next_payment_date.php

<?php

// Copy from below here...

/**
 * Send users' next payment date to Mailchimp.
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpromc_update_on_subscription_payment_completed( $morder ) {
	if ( ! function_exists( 'pmpromc_getAPI' ) ) {
		// PMPro Mailchimp is not active.
		return;
	}

	$user = get_userdata( $morder->user_id );

	// Get API and bail if we can't set it.
	$api = pmpromc_getAPI();
	if ( empty( $api ) || empty( $user ) ) {
		return;
	}

	// Get all audiences.
	$audiences = $api->get_all_lists();

	if ( ! empty( $audiences ) ) {
		foreach ( $audiences as $audience ) {
			// Check for member.
			$member = $api->get_listinfo_for_member( $audience->id, $user );
			if ( isset( $member->status ) ) {
				// Update the user's merge fields.
				pmpromc_add_audience_member_update( $user->ID, $audience->id, $member->status );
			}
		}
	}
}
add_action( 'pmpro_subscription_payment_completed', 'my_pmpromc_update_on_subscription_payment_completed' );

function my_pmpro_mailchimp_listsubscribe_fields_nextpayment( $fields, $user ) {
	// Get user's next payment date.
	$next_payment_date = pmpro_next_payment( $user->ID );

	if ( ! empty( $next_payment_date ) ) {
		$fields['NEXTPAYMNT'] = date( 'Y-m-d', $next_payment_date );
	} else {
		$fields['NEXTPAYMNT'] = 'None';
	}

	return $fields;
}
add_action( 'pmpro_mailchimp_listsubscribe_fields', 'my_pmpro_mailchimp_listsubscribe_fields_nextpayment', 10, 2 );

/*
 * Creates an NEXTPAYMNT merge field in MailChimp.
 */
function my_pmpro_mailchimp_merge_field_nextpayment( $merge_fields ) {
	$merge_fields[] = array(
		'name' => 'NEXTPAYMNT',
		'type' => 'text'
	);
	return $merge_fields;
}
add_filter( 'pmpro_mailchimp_merge_fields', 'my_pmpro_mailchimp_merge_field_nextpayment' );