Add an email tag for a custom Charitable campaign field

edd-function.php

<?php
/**
 * In Easy Digital Downloads, you can add your own email tags to include 
 * in purchase receipts & notifications.
 *
 * In this example, we add a tag that allows you to display the value of
 * a custom 'campaign_email_content' field in Charitable. This is an optional
 * follow-up to a tutorial published on our blog.
 *
 * @see https://www.wpcharitable.com/tutorial-how-to-add-campaign-specific-content-to-the-donation-receipt-email/
 */

/**
 * Register the tag.
 *
 * Note that the first parameter passed to the edd_add_email_tag() function 
 * will be the name of the tag. 
 * 
 * @see   http://docs.easydigitaldownloads.com/article/497-edd-add-email-tag
 *
 * @param int $payment_id
 */
add_action( 'edd_add_email_tags', function( $payment_id ) {

	/**
	 * Add the tag.
	 *
	 * This function receives three parameters:
	 *
	 * $tag_name        The name of the tag that you will use to display it in your email. So in this case, you 
	 *                  would use this tag in your email like this: {campaign_email_content}
	 * $tag_description A description of what the tag shows.
	 * $callback        A callback function that is responsible for getting the value of the tag. This receives a
	 *                  $payment_id variable as a parameter.
	 */
	edd_add_email_tag( 'campaign_email_content', 'Display campaign-specific text', function( $payment_id ) {
		$output = '';
		
		/**
		 * Get all campaign donations linked to a particular payment.
		 */
		$campaign_donations = Charitable_EDD_Payment::get_instance()->get_campaign_donations_through_payment( $payment_id );

		if ( ! $campaign_donations ) {
			return $output;
		}

		/**
		 * Get the campaign that received the donation.
		 */
		$campaign_id = current( $campaign_donations )->campaign_id;
		$campaign = charitable_get_campaign( $campaign_id );

		/**
		 * Set the `value` of our field to the campaign's email content.
		 */
		$output = $campaign->get( 'campaign_email_content' );
		
		return $output;
	} );
} );