Shows a simplified way to add a custom email value tag for use in a Participants Database email template

pdb-custom-email-tag.php

<?php

/**
 * Plugin Name: PDb Custom Email Tag
 * Version: 0.1
 */

// this is a proof-of-concept example

add_filter( 'pdb-template_email_tag_map', 'xnau_add_custom_email_tag', 10, 2 );

/**
 * adds the tag to the tag map
 *
 * @param array $tag_map as $tagname => $value
 * @param string $context usually the template name but identifies the current context of 
 *                        the email template, you can use that to determine whether to add 
 *                        the tag or not
 * @return array
 */
function xnau_add_custom_email_tag( $tag_map, $context ) {
  
  // we're adding a tag "custom"
  // this tag would be used in the email template as [custom]
  // and would be replaced by the value in the email
  $tagmap['custom'] = 'custom value';
  
  return $tag_map;
}