Get a custom list of all Drupal terms on an entity. Useful when you have several term reference fields on a single entity, and want to display them all together. _get_entity_terms() written by Drupal user HydroZ.

custom.module

<?php

/**
 * Example: get all terms attached to a given taxonomy term.
 * 
 * Can be done with any entity.
 */
function custom_example($term) {
  $related_term_links = array();
  $related_tids = _get_entity_terms('taxonomy_term', $term->vocabulary_machine_name, $term);
  $related_terms = taxonomy_term_load_multiple($related_tids);
  foreach ($related_terms as $related_term) {
    $related_term_links[] = l($related_term->name, 'taxonomy/term/' . $related_term->tid);
  }
		
  if (!empty($related_term_links)) {
    $output = '<div class="related-terms"><span class="label">Related Topics:</span> ' . implode(' | ', $related_term_links) . '</div>';
    return $output;
  }
}

/**
 * Fetchs all Taxonomy Term IDs from Entity Object. All fields of field type "taxonomy_term_reference" will be included.
 *
 * @param String $entity_type
 * @param $string $bundle
 * @param Object $entity
 *
 * @return array
 *   Array with tids of entity
 */
function _get_entity_terms($entity_type, $bundle, $entity) {
  $tids = array();
  foreach (field_info_field_map() as $field_name => $field_info) {
    if ($field_info['type'] <> "taxonomy_term_reference") {
      continue;
    }
    if (array_key_exists($entity_type, $field_info['bundles'])) {
      if (in_array($bundle, $field_info['bundles'][$entity_type])) {
        if (isset($entity->{$field_name})) {
          $values = field_get_items($entity_type, $entity, $field_name);
          foreach ((array)$values as $tid) {
            $tids[] = $tid['tid'];
          }
        }
      }
    }
  }
  return array_unique(array_filter($tids));
}