Just two simple questions: What are must-use (mu) plugins and what are drop-ins? I’ll answer both right away:
Must Use Plugins
The MU or Must-Use Plugins is a principle that has moved from WordPress MU into WordPress core long before the 3.0 merge. These are standard wordpress plugins files, that are loaded regardless if they have been configured in your blogs backend or not. They only need to be placed in a specific directory, the mu-plugins folder:
wp-content/mu-plugins. That feature is available since WordPress 2.8 if I remember correctly. At least since 2.8 it can be configured with the
The plugin files do not need to contain plugin headers, so you just need to create a PHP file and that’s all.
This term is not that well known. Some core functionality of the WordPress core can be replaced by so called Drop-Ins. Those are PHP files on specific locations that get included if they exist. The inclusion is done for a specific task, for example
db.php get’s loaded to replace the default PHP database class. So you can replace it with one that is faster, more stable and secure for example. Drop-Ins exist since various versions, depending on them. The Multi-Site since 3.0 naturally.
List of all WordPress Drop-Ins
|File||Type of Plugin||Loaded||Context|
|advanced-cache.php||Advanced caching plugin.||on
|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
|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|
Source: Data taken from global function
All files need to be placed in the wordpress content directory (
wp-content by default, defined in constant WP_CONTENT_DIR). Drops-Ins do not need to be full plugin files, they can be just standard PHP files.
WordPress 3.0 will ship with a better plugin listing by the way. It reveals which of those non-standard-plugin extensions are loaded.