WPEngine - Cache key override - You need this if you want to run a secondary ( or multisite ) in a subdirectory on WP Engine

wpeCacheFix.php

<?php
/**
* Author:  Chase C. Miller
* Working Date: 2017-01-11
* Custom cache key for WPEngine to bypass object caching problems that can happen when you install a secondary or multisite on WPEngine.
* It should be installed as a mu-plugin to catch early activation.
**/

global $wp_object_cache;

if ($wp_object_cache && get_class($wp_object_cache) == 'WP_Object_Cache') {
  // Change a-unique-key to a unique identifier for your site.
  $wp_object_cache->customer = 'a-unique-key';
}