WordPress Must Use Plugins

WordPress must use plugins are plugins that are always loaded and cannot be deactivated within the WordPress admin.

Setup

A bit of a hidden feature, must use plugins require a little setup initially:

• First, you will need to create an mu-plugins directory within the WordPress content directory. The mu-plugins directory will sit alongside your normal plugins directory, normally at wp-content/mu-plugins unless you’ve moved or renamed the WordPress content directory.
• Then, any files you upload into that directory will be loaded automatically. Just be aware that dropping a folder into mu-plugins won’t do anything, only standalone files will load.

The Pros

• Must use plugins are always on and cannot be disabled within the admin area.
• Must use plugins load before normal plugins.

The Cons

• Must use plugins will never check for or show available updates.
• Must use plugins do not trigger plugin activation hooks.

Tips & Tricks

• Dropping a file into the mu-plugins directory means that code runs automatically, without having to log into the admin. This comes in handy when you need to do things like force a WordPress admin user.
• If you want to load a normal plugin as a must use plugin, just drop the plugin folder into the mu-plugins directory and then create a plugin-loader.php file (any name will do) that does a require for the main plugin file.
• Files in the mu-plugins directory load alphabetically. If you need to manually control the load order, you can simply rename the files.
• You can move or rename the mu-plugins directory by setting these constants in your wp-config.php file:

define( 'WPMU_PLUGIN_DIR', __DIR__ . '/wp-content/required-plugins' );
define( 'WPMU_PLUGIN_URL', 'http://mydomain.com/wp-content/required-plugins' );