Creates a {donation_id} for GiveWP Emails

donation_id_tag.php

<?php
function donation_id_tag( $email_tags ) {

	$new_email_tag = array(
		'tag'         => 'donation_id',
		'description' => esc_html__( 'This tag outputs the donation ID', 'give' ),
		'function'    => 'get_donation_id',
		'context'     => 'general', // Context can be general, donor, form or donation
	);

	array_push( $email_tags, $new_email_tag );

	return $email_tags;
}
add_filter( 'give_email_tags', 'donation_id_tag' );

function get_donation_id( $tag_args ) {
	$output       = '';
	$donation_id      = $tag_args['payment_id'];
	$output       = $donation_id;
	return $output;
}