How to Conditionally Disable Object Cache Drop-in

I recently faced an interesting challenge: a site has a single primary domain name and several secondary domain names. The site(s) share the same codebase and database, but have several domain names. The site used an object cache drop-in. Site administrators used only the primary domain for posting. All the administrative operations triggered cache updates only for the primary domain; the mirrors on the secondary domains could show stale data. Site owners somehow needed to disable the object cache for all domains except the primary one.

Surprisingly, WordPress has the enable_loading_object_cache_dropin filter that controls whether to enable loading of the object-cache.php drop-in. That filter comes with one caveat, though:

This filter runs before it can be used by plugins. It is designed for non-web run-times. If false is returned, object-cache.php will never be loaded.

The above means there is no way to write a plugin, including a “must-use” plugin, to control the loading of the drop-in.

However, there is a workaround (or a hack, if you will) to add a filter in your wp-config.php.

The wp_debug_mode() function gives a hint on how to do this:

* This filter runs before it can be used by plugins. It is designed for
* non-web run-times. Returning false causes the `WP_DEBUG` and related
* constants to not be checked and the default PHP values for errors
* will be used unless you take care to update them yourself.
*
* To use this filter you must define a `$wp_filter` global before
* WordPress loads, usually in `wp-config.php`.
*
* Example:
*
*     $GLOBALS['wp_filter'] = array(
*         'enable_wp_debug_mode_checks' => array(
*             10 => array(
*                 array(
*                     'accepted_args' => 0,
*                     'function'      => function() {
*                         return false;
*                     },
*                 ),
*             ),
*         ),
*     );

Of course, we can employ the same technique for the `enable_loading_object_cache_dropin` filter. All we need is to place this piece of code into `wp-config.php` above the `require_once(ABSPATH . ‘wp-settings.php’);` line:

$GLOBALS['wp_filter'] = [
    'enable_loading_object_cache_dropin' => [
        10 => [
            [
                'accepted_args' => 1,
                'function'      => function ( $load ) {
                    /* Here comes the logic that determines whether WP should load object-cache.php */                
                    return $load;
                },
            ],
        ],
    ],
];