Add Gift Aid note to recurring orders if user has opted in.

my-gift-aid-recurring.php

<?php   // do not copy this line.
/**
 * This recipe Adds Gift Aid note to recurring orders if user has opted in.
 * 
 * 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 pmproga_add_gift_aid_to_recurring_orders( $order ) {
	if ( empty( $order ) || empty( $order->user_id ) ) {
		return;
	}

	// Check user's stored Gift Aid preference.
	$gift_aid = get_user_meta( $order->user_id, 'gift_aid', true );

	// Only append if not already in notes.
	if ( strpos( $order->notes, 'Gift Aid:' ) === false ) {
		if ( $gift_aid == 1 ) {
			$order->notes .= "Gift Aid: Yes\n";
		} else {
			$order->notes .= "Gift Aid: No\n";
		}
		$order->saveOrder();
	}
}
add_action( 'pmpro_added_order', 'pmproga_add_gift_aid_to_recurring_orders' );