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To install a must-use (MU) plugin, create wp-content/mu-plugins if it does not exist, place the plugin’s PHP file directly inside it, and reload WordPress. MU plugins load automatically on every request, so there is no Activate button. In the dashboard, you can inspect them under Plugins → Must-Use, but you disable one by renaming, moving, or deleting its file through SFTP, SSH, or your host’s file manager.
This guide shows the exact folder layout, a safe installation workflow, the loader pattern required for multi-file plugins, load-order rules, host-specific exceptions, and recovery steps when an MU plugin does not appear or causes an error.
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A must-use plugin is a PHP plugin that WordPress loads automatically from a dedicated directory before ordinary plugins and themes. The default directory is /wp-content/mu-plugins/. WordPress does not create this directory in a standard installation, so you normally create it yourself.
The automatic loading model is useful for code that must always run, such as security policies, environment-specific configuration, maintenance controls, or a small site-wide filter. It also removes a common failure point: an administrator cannot accidentally deactivate the code from the Plugins screen.
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| Characteristic | Normal plugin | Must-use plugin |
|---|---|---|
| Default location | wp-content/plugins/ |
wp-content/mu-plugins/ |
| Activation | Installed, then activated in the dashboard | Loaded automatically when its PHP file is found |
| Dashboard control | Can be activated, deactivated, and usually updated in the dashboard | No Activate or Deactivate control; file-system access is required |
| Load timing | After must-use plugins | Before normal plugins and themes |
| Failure impact | Usually limited to the plugin’s code path | A syntax or runtime failure can affect every request, including the dashboard |
| Activation hooks | Activation and deactivation hooks can run | Activation hooks do not run |
MU plugins are different from drop-ins. A drop-in is a specially named file, such as an object-cache implementation, that replaces or extends a specific core subsystem. It follows its own loading rules and should not be placed in the MU directory simply because it is “always on.”
Prepare before uploading code
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- SFTP access with the server name, username, password or key, and port supplied by your host.
- SSH access and a shell account with permission to edit the WordPress installation.
- Your hosting control panel’s file manager.
Make a backup of the site files and database, or use a staging copy, before installing unfamiliar code. A malformed MU file is loaded on every request, so a single missing semicolon can produce a site-wide fatal error. Confirm that the code supports the PHP version used by the site and that the account uploading it can read the file.
Keep a copy of the original file and record who installed it, why it exists, and how it is deployed. MU plugins do not receive the normal plugin update workflow, so maintenance is your responsibility.
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Install a single-file MU plugin
- Connect to the WordPress files. Open the directory containing
wp-admin,wp-includes, andwp-content. - Open
wp-content. Look for a directory namedmu-plugins. WordPress installations commonly lack it. - Create the directory when necessary. Create a directory named exactly
mu-plugins, using lowercase letters and the hyphen. The resulting default path is/wp-content/mu-plugins/. - Create or obtain the PHP file. For example, save the following as
site-policy.php:
<?php
/**
* Plugin Name: Site Policy MU Plugin
* Description: Adds a site-wide policy example.
*/
add_action('init', function () {
// Put code that should run on every request here.
});- Upload the file directly into the MU directory. The path must be
/wp-content/mu-plugins/site-policy.php, not/wp-content/mu-plugins/site-policy/site-policy.phpunless a top-level loader is present. - Check the dashboard. Sign in to WordPress and open Plugins → Must-Use. A valid plugin header helps WordPress display a readable name and description.
- Exercise the feature. Visit the front end and the relevant administrative screen, then inspect the PHP error log for warnings or fatal errors. Do not assume that appearing in the Must-Use list proves the plugin’s business logic works.
The header comment is not what activates the file. WordPress loads a top-level PHP file because of its location. The header makes the item easier to identify in the dashboard and for future administrators.
Install a multi-file MU plugin with a loader
WordPress automatically scans only PHP files directly inside the MU directory. It does not recursively load PHP files from subdirectories. Organize a larger plugin in its own subdirectory, then add one top-level loader file that explicitly includes the main code.
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wp-content/
└── mu-plugins/
├── load-site-policy.php
└── site-policy/
├── site-policy.php
└── includes/
└── rules.phpThe top-level load-site-policy.php can contain:
<?php
/**
* Plugin Name: Site Policy Loader
*/
require_once __DIR__ . '/site-policy/site-policy.php';The loader is the only file WordPress needs to discover. The required file can then require additional files inside the subdirectory. Use require_once to prevent duplicate declarations if another component reaches the same file.
Do not leave development copies, editor backups, or old versions ending in .php in the top level. Every top-level PHP file is a candidate for loading on every request, including a file that was never intended to be active.
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Understand MU plugin loading rules
Only top-level PHP files load automatically
A file such as mu-plugins/example.php loads automatically. A file such as mu-plugins/example/example.php is ignored until a top-level loader requires it. This rule is the reason a multi-file plugin needs a loader at the root of mu-plugins.
Files load on every request
Front-end pages, REST requests, cron requests, login requests, and dashboard pages all load the MU files. Keep the top-level code lightweight, avoid expensive database work on every request, and put conditional logic around operations that only belong on particular screens or hooks.
Alphabetical order matters when code depends on order
WordPress loads MU files alphabetically. If one file must load before another, teams often prefix filenames with values such as 00- and 10-. This is a practical naming convention, not a substitute for explicit dependencies, and it is not a requirement that every site must follow.
Activation and deactivation hooks do not run
Because there is no activation event, code attached to register_activation_hook() will not initialize an MU plugin. Create required options, tables, or scheduled events through an explicit setup routine, a one-time command, or an administrative migration instead. Runtime hooks such as init, plugins_loaded, and feature-specific filters can still be used when their timing is appropriate.
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Verify that WordPress loaded the plugin
- Open Plugins → Must-Use in the WordPress dashboard.
- Confirm that the expected file or plugin header appears.
- Test the behavior that the plugin is meant to provide.
- Review the PHP and WordPress logs after the test.
The Must-Use screen is an inventory, not an activation panel. There is no switch to turn an item on. A file can also execute even when its display name is confusing or its header is missing, so use the file path and server logs when investigating.
Disable, remove, or replace an MU plugin
There is no Deactivate button for an MU plugin. Use SFTP, SSH, or the host’s file manager:
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- Rename the PHP file so it no longer ends in
.php, for examplesite-policy.php.disabled, or move it outsidemu-plugins. - Reload the site and verify that the feature is gone and that the error has cleared.
- Delete the file only after confirming that no other code depends on it.
Renaming is safer than immediate deletion because it provides a quick rollback. When replacing a file, upload the new version under a temporary name, check it for syntax errors, and then perform a controlled rename so the old copy remains available until the new one is confirmed.
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| Symptom | Likely cause | What to check |
|---|---|---|
| Nothing appears under Must-Use | Wrong directory, wrong site, or a host-managed path | Confirm the active installation’s wp-content path, spelling of mu-plugins, and whether the host documents a different directory. |
| The file appears but the feature does nothing | Code is hooked to the wrong action, has a conditional that never matches, or depends on activation-time setup | Check the hook timing, conditions, logs, and any setup routine. Do not rely on an activation hook. |
| The whole site returns a fatal error | PHP syntax error, missing required file, incompatible PHP code, or an exception during an early hook | Use the host file manager or SSH to rename the MU file, inspect the PHP error log, correct the problem on a staging copy, and upload the repaired version. |
| A subfolder file is ignored | There is no top-level loader | Add a top-level PHP loader with a correct require_once path. |
| Changes appear not to take effect | Object, page, CDN, or opcode cache | Clear the relevant cache after confirming that the edited file is on the server handling the request. |
| The plugin works on one environment but not another | Different PHP versions, paths, permissions, or host conventions | Compare PHP versions, absolute paths, file ownership, and the host’s MU directory policy. |
Check for a path mistake
It is easy to upload into a similarly named staging directory, a nested wp-content, or the wrong site in a multisite or multi-instance server. Locate the wp-config.php that belongs to the running site, then trace its wp-content directory and verify the file’s absolute path.
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Check the PHP file itself
Ensure the file begins with a PHP opening tag, contains no accidental characters before that tag, and has balanced braces, parentheses, and quotes. A file can be listed yet fail during execution, so the PHP log is more useful than the dashboard label for runtime errors.
Check dependencies
An MU plugin loads before normal plugins. If it calls a function supplied by a normal plugin before that dependency has loaded, guard the call or attach it to a later hook. For a hard dependency, document the required plugin and fail with a clear logged message rather than producing an unexplained fatal error.
Custom MU directories and managed hosting
The default path is not universal. WordPress can use a directory defined by the WPMU_PLUGIN_DIR constant. If the installation defines that constant, the custom directory takes precedence over the default location. In a self-managed installation, the constant is normally defined in wp-config.php before WordPress loads; follow the host’s documentation when configuration is generated automatically.
Some managed platforms maintain their own conventions. WordPress VIP documents a /client-mu-plugins directory at the root of the site’s code repository. Other hosts may install a provider-owned MU plugin or restrict direct uploads. A file placed in the standard directory will not be used if the platform’s bootstrap process points WordPress elsewhere, so check the platform’s deployment instructions first.
Security and maintenance practices
- Use version control. Store MU code in the same repository and review process as other production code.
- Limit privileges. Give the plugin only the capabilities and database access it needs.
- Avoid hidden administration. Document every behavior, scheduled task, option, endpoint, and external request the file creates.
- Log safely. Never write passwords, access tokens, or personal data to debug logs.
- Test on staging. Exercise front-end, REST, cron, login, and administrator requests because all of them load MU files.
- Plan updates. There is no normal plugin update notice or automatic update path for custom MU code. Deploy tested files deliberately and retain a rollback copy.
- Remove abandoned code. An unused file still executes, consumes resources, and expands the attack surface.
When to choose an MU plugin instead of a normal plugin
Choose an MU plugin when the code must be present regardless of dashboard activation state, when administrators should not be able to switch it off accidentally, or when deployment is managed as infrastructure code. Choose a normal plugin when site owners need dashboard activation, updates, settings screens, or a conventional install-and-remove lifecycle.
Quick Recap
| Use case | Better fit | Reason |
|---|---|---|
| Site-wide policy that must always run | MU plugin | It loads automatically and cannot be deactivated in the dashboard. |
| Feature that editors may turn on or off | Normal plugin | The dashboard provides activation control. |
| Code requiring an activation migration | Normal plugin, or an MU plugin with a separately managed migration | MU plugins do not trigger activation hooks. |
| Host-controlled bootstrap or deployment integration | Host-specific MU location | The platform may load files from a directory other than wp-content/mu-plugins. |
| Core replacement such as object caching | Drop-in | Drop-ins have special filenames and loading behavior. |
Quick installation checklist
- Confirm you are editing the active WordPress installation.
- Back up the site or use staging.
- Create
wp-content/mu-pluginsif the host has not supplied another path. - Put single-file plugins directly in that directory.
- For multi-file plugins, add a top-level loader using
require_once. - Check Plugins → Must-Use and test the actual behavior.
- Remember that every PHP file there runs on every request.
- To disable the plugin, rename or move its file; do not look for a Deactivate button.
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