Drop-ins are single-file PHP overrides placed in the /wp-content/ directory that replace or extend specific internal WordPress subsystems before most of the CMS initializes. Unlike plugins, drop-ins load extremely early in the WordPress boot sequence, making them the correct solution for replacing core functions that plugins cannot override. Named drop-in files WordPress recognizes include: object-cache.php (replaces the default non-persistent object cache with Redis or Memcached), advanced-cache.php (implements a custom full-page caching layer, invoked when WP_CACHE is true in wp-config.php), db.php (replaces the wpdb database class), and maintenance.php (customizes the maintenance mode page). Because drop-ins operate outside the plugin activation system, they must be managed manually or by the hosting platform — WP Engine and other managed hosts use object-cache.php drop-ins to provide Redis-backed persistent caching automatically.
Glossary entry