Wordpress + Memcached: How to set it up properly

• Self-hosted WordPress Instance
• Memcached Redux Plugin installed (not activated)
• Memcached
• PHP5 Memcached Module

• WordPress Self-Hosted
• Debian Server OS (Example: Ubuntu)

1. Download the Memcached Redux plugin and put it in your /wp-content/plugins/ directory

2. Don’t Activate the plugin (never activate the plugin)

3. copy the object-cache.php file from /wp-content/plugins/memcached-redux/ directory to the wp-content directory.
4. Open your wp-config.php file for editing
5. In the “Authentication Unique Keys and Salts” section place the following line below the $table_prefix definition (required for multi-install servers):

define('WP_CACHE_KEY_SALT', md5( DB_NAME . $table_prefix . __FILE__ ) );

6. Save the file
7. From your server’s command line install memcached

sudo apt-get install memcached

8. From your server’s command line install php5-memcached

If you have php5:

sudo apt-get install php5-memcached

If you have php7:

sudo apt-get install php-memcached

9. Restart apache

sudo service apache2 restart

The plugin serves as a placeholder for the object-cache.php file. It can also serve as a way for you to know if there are updates in the future. If you do activate it you’ll receive the following PHP Fatal error and your site and/or admin will display a White Screen of Death (WSOD).

PHP Fatal error:  Cannot redeclare wp_cache_add()

This is because when a plugin is activated WordPress will execute the code contained in it’s main php file. Copying the object-cache.php to the wp-content/ directory is all that’s needed to make the plugin work.

This step is good for anyone to do, but is highly recommended for servers that host multiple WordPress installations that are using memcached.

An example of what could happen if you don’t take the step on a multi-install server is any blog using a common database prefix (default is wp_) could take on portions of other sites using the same prefix.

I haven’t done extensive testing, but it appears that sites that use different database prefixes are likely ok to exclude this extra definition, but better to be safe than sorry.

I’m neither the plugin author or a contributing developer on the memcached project, but I am an enthusiastic user. That said, the information above was gleaned from multiple sources and I couldn’t find a comprehensive guide on how to fully install and use the Memcached Redux plugin.