WordPress 3.4 Upgrade Issue


I was updating a site today to WordPress 3.4, and ran into a strange server error after performing the update that prevented the site from working at all and also prevented the database from updating.

Call to undefined function wp_cache_add_non_persistent_groups()

Took a bit of investigation to figure it out. The issue was that the site was using the ancient wp_cache plugin.

How to fix:

  1. Remove the line define('WP_CACHE', true); from wp-config.php
  2. Delete advanced_cache.php from the wp-content folder
  3. Delete wp_cache_config.php from the wp-content folder
  4. Delete the wp-cache folder from the plugins folder