<?php

class Example extends DataObject {

  private static $db = ['Option' => 'Varchar'];

  private static $defaults = ['Option' => 'cats'];

  static $options = ['cats' => 'Cats', 'dogs' => 'Doggies'];

  public function getCMSFields() {
      $fields = parent::getCMSFields();
      $fields->replaceByName(
        'Option',
        DropdownField::create('Option')
          ->setSource(static::$options);
      return $fields;
  }

}