drupal/core/modules/options/tests/options_test.module

<?php

/**
 * @file
 * Helper module for the List module tests.
 */

use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Field\FieldDefinitionInterface;

/**
 * Allowed values callback.
 */
function options_test_allowed_values_callback(FieldDefinitionInterface $field_definition, EntityInterface $entity) {
  $values = array(
    'Group 1' => array(
      0 => 'Zero',
    ),
    1 => 'One',
    'Group 2' => array(
      2 => 'Some <script>dangerous</script> & unescaped <strong>markup</strong>',
    ),
  );

  return $values;
}

/**
 * An entity-bound allowed values callback.
 */
function options_test_dynamic_values_callback(FieldDefinitionInterface $field_definition, EntityInterface $entity, &$cacheable) {
  $cacheable = FALSE;
  // We need the values of the entity as keys.
  $uri = $entity->uri();
  return drupal_map_assoc(array(
    $entity->label(),
    $uri['path'],
    $entity->uuid(),
    $entity->bundle(),
  ));
}