Add !!tax!! variable for Paid Memberships Pro email templates

mypmpro-add-custom-field-to-email-template.php

<?php
/**
 * Adds an email variable !!tax!! to Paid Memberships Pro emails.

 * Use the Email Templates Admin Editor to add !!tax!! to your email templates.
 * Follow this guide to add this code to your site: https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
 * Difficulty: Easy
 */
function my_pmpro_email_variable( $data, $email ) {

	$order = new MemberOrder( $data['invoice_id'] );
	$data['tax'] = get_user_meta( $order->user_id, 'tax_number', true );

	return $data;
}
add_filter( 'pmpro_email_data', 'my_pmpro_email_variable', 10, 2 );