WordPress drop-ins are custom php Must-Use files related to some core files such as db.php, advanced-cache.php and object-cache.php that you can place in the wp-content directory to override the core file with new functionality.
List of WordPress Drop-Ins
| File | Type of Plugin | Loaded | Context |
|---|---|---|---|
| advanced-cache.php | Advanced caching plugin. | on WP_CACHE value |
Single |
| db.php | Custom database class | always | Single |
| db-error.php | Custom database error message | on error | Single |
| install.php | Custom install script | on install | Single |
| maintenance.php | Custom maintenance message | on maintenance | Single |
| object-cache.php | External object cache | always | Single |
| sunrise.php | ‘Executed before Multisite is loaded | on SUNRISE value |
Multi |
| blog-deleted.php | Custom blog deleted message | on deleted blog | Multi |
| blog-inactive.php | Custom blog inactive message | on inactive blog | Multi |
| blog-suspended.php | Custom blog suspended message | on archived or spammed blog | Multi |
This list is extracted from global function _get_dropins() in wp-admin/includes/plugin.php.