What are WordPress Drop-in Plugins?

WordPress drop-in plugins are a hidden feature of WordPress used to replace, add, or enhance a limited set of advanced core WordPress features. They are only created by developers or other regular plugins. WordPress drop-in plugins are not regular plugins.

Drop-in plugins are special files that each have a singular and unique purpose within WordPress. For example, you can use a drop-in plugin to replace the core WordPress db.php file and it’s wpdb class. You could also use the advanced-cache.php WordPress drop-in to add “Advanced Caching” to your website (how the caching works is entirely up to the developer, there are no training wheels with drop-ins). Basically each drop-in has its own purpose and each drop-in works in a different way.

Again, to be clear. Even though drop-in plugins appear on the WordPress plugins page, drop-in plugins are not like regular WordPress plugins that you install through the WordPress admin. Nor are they anything like must-use plugins.

All WordPress Drop-In Plugins

File	Description	Loaded	Type
advanced-cache.php	Advanced caching plugin.	If WP_CACHE is true	Normal
db.php	Custom database class.	on load	Normal
db-error.php	Custom database error message.	on error	Normal
install.php	Custom installation script.	on install	Normal
maintenance.php	Custom maintenance message.	on maintenance	Normal
object-cache.php	External object cache.	on load	Normal
php-error.php	Custom PHP error message.	on error	Normal
fatal-error-handler.php	Custom PHP fatal error handler.	on error	Normal
sunrise.php	Executed before Multisite is loaded.	If SUNRISE is true	Multisite
blog-deleted.php	Custom site deleted message.	on deleted blog	Multisite
blog-inactive.php	Custom site inactive message.	on inactive blog	Multisite
blog-suspended.php	Custom site suspended message.	on archived or spammed blog	Multisite

Frequently Asked Questions About WordPress Drop-in Plugins

When were drop-in plugins added to WordPress source? Drop-in plugins were added to WordPress in version 3.0.

When do drop-in plugins get loaded? Most drop-in plugins run before any other regular plugin or MU plugin. The table above also demonstrates the run time of each drop-in.

Where can I see my active WordPress drop-in plugins? Drop-in plugins can be seen in the WordPress admin on the plugins page under Plugins | Drop-in.

How can I activate, deactivate, or uninstall drop-in plugins from the WordPress plugins dashboard? You can not manage drop-in plugins from the WordPress admin, drop-ins can only be managed on your server.

How to Get Started with WordPress Drop-in Plugins

If you wanted to start using drop-in plugins you will not find any organized documentation on wordpress.org about them. This is not the answer you were hoping to hear but it is true.

To get started you will need to look through the WordPress source code and start searching Google. Drop-ins certainly are a hidden feature of WordPress for advanced development.

However, there are a few getting started pointers I can offer if you want to dig deeper.

1. Browse through the WordPress core and you will find the _get_dropins() function that lists all of the available drop-ins you could install.
2. To install a drop-in plugin you simply add it directly in the wp-content folder.
3. When researching a drop-in plugin be specific. Search for the single file that is relevant to your needs (see the list of files below).
4. Find other regular plugins, like WP Rocket, that install drop-ins and see how they use them.

Below I’ve listed the contents of the _get_dropins() function.

/**
 * Returns drop-ins that WordPress uses.
 *
 * Includes Multisite drop-ins only when is_multisite()
 *
 * @since 3.0.0
 * @return array Key is file name. The value is an array, with the first value the
 *  purpose of the drop-in and the second value the name of the constant that must be
 *  true for the drop-in to be used, or true if no constant is required.
 */
function _get_dropins() {
	$dropins = array(
		'advanced-cache.php'      => array( __( 'Advanced caching plugin.' ), 'WP_CACHE' ), // WP_CACHE
		'db.php'                  => array( __( 'Custom database class.' ), true ), // auto on load
		'db-error.php'            => array( __( 'Custom database error message.' ), true ), // auto on error
		'install.php'             => array( __( 'Custom installation script.' ), true ), // auto on installation
		'maintenance.php'         => array( __( 'Custom maintenance message.' ), true ), // auto on maintenance
		'object-cache.php'        => array( __( 'External object cache.' ), true ), // auto on load
		'php-error.php'           => array( __( 'Custom PHP error message.' ), true ), // auto on error
		'fatal-error-handler.php' => array( __( 'Custom PHP fatal error handler.' ), true ), // auto on error
	);

	if ( is_multisite() ) {
		$dropins['sunrise.php']        = array( __( 'Executed before Multisite is loaded.' ), 'SUNRISE' ); // SUNRISE
		$dropins['blog-deleted.php']   = array( __( 'Custom site deleted message.' ), true ); // auto on deleted blog
		$dropins['blog-inactive.php']  = array( __( 'Custom site inactive message.' ), true ); // auto on inactive blog
		$dropins['blog-suspended.php'] = array( __( 'Custom site suspended message.' ), true ); // auto on archived or spammed blog
	}

	return $dropins;
}

I wish you all the best and hope this article helps you with your WordPress drop-ins plugin journey.