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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsTo install a WordPress must-use (MU) plugin, create wp-content/mu-plugins, place the plugin’s PHP file directly inside it, and load the site. WordPress loads top-level PHP files in that directory automatically, so there is no Activate button. This guide shows how to build the folder, install single- and multi-file plugins, verify loading, control load order, recover from errors, and remove an MU plugin safely.
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What a must-use plugin is
A must-use plugin is a WordPress plugin that is loaded automatically from a special directory. It is intended for code that must run on every request and should not depend on an administrator remembering to activate it. Typical uses include enforcing site-wide settings, registering functionality for a managed installation, adding security or operational rules, and loading code that needs to run before ordinary plugins and themes.
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WordPress does not normally create the MU directory for a new site. You create it under wp-content. The conventional path is:
/wp-content/mu-plugins/
Any PHP file placed directly in that directory is loaded automatically. There is no activation step, and the administrator cannot deactivate it from the Plugins screen. The absence of an activation switch is the defining operational difference, not a special PHP language or a different plugin API.
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Must-use, normal, and drop-in plugins compared
| Type | Usual location | How it starts running | Admin activation control | Important limitation |
|---|---|---|---|---|
| Normal plugin | wp-content/plugins |
Install, then activate it in WordPress | Can be activated and deactivated in the dashboard | Code that depends on activation hooks can run during activation |
| Must-use plugin | wp-content/mu-plugins |
Automatically loaded when WordPress starts | No Activate or Deactivate control | Activation hooks do not run; file changes must be made on the server |
| Drop-in plugin | Specific files in wp-content, such as object-cache.php |
WordPress detects a specially named replacement file | Managed according to that drop-in’s behavior | It replaces or extends a core subsystem rather than behaving like a normal plugin |
Drop-ins and MU plugins are often mentioned together because both are loaded outside the ordinary activation workflow, but they are not interchangeable. A file named object-cache.php, for example, is a cache drop-in; putting an unrelated plugin in the wrong location will not convert it into a drop-in.
Before you create the directory
You need a way to edit the site’s files and enough access to write inside the WordPress installation. Use one of these methods:
- SFTP: connect with the host, port, username, password or key, and remote path supplied by your host.
- SSH: connect to the server and use shell commands or an editor, provided your account can write to the WordPress directory.
- Host file manager: use the file manager supplied by your hosting control panel.
Make a backup or commit the change to your deployment system before installing code that will run on every request. A PHP parse error in an MU plugin can affect the front end, the login page, and the administration area at the same time. Keep a copy of the original file so you can restore it quickly.
Confirm that you are editing the production site’s actual WordPress root. The root contains files such as wp-config.php, wp-admin, wp-includes, and wp-content. Creating mu-plugins beside, rather than inside, wp-content will not work unless the site has deliberately configured a custom directory.
Create the standard mu-plugins directory
On a self-managed WordPress installation, follow these steps:
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- Connect to the site over SFTP, SSH, or the host’s file manager.
- Open the WordPress installation’s
wp-contentdirectory. - Create a new directory named exactly
mu-plugins. Use lowercase letters and the hyphen shown; do not name itmu_plugins,must-use, ormust-use-plugins. - Confirm the resulting path is
/wp-content/mu-plugins/. - Upload the MU plugin’s PHP file directly into that directory.
The directory may be empty until you add a file. Its absence on a new WordPress site is normal; creating it is part of the installation process.
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A single-file plugin is the simplest arrangement. If the file is called my-plugin.php, upload it to:
/wp-content/mu-plugins/my-plugin.php
A minimal file can contain a plugin header and ordinary WordPress hooks:
<?php
/**
* Plugin Name: Site Rules
*/
add_action('init', function () {
// Site-wide code goes here.
});The header is useful for identifying the file in the dashboard, but it does not create an activation workflow. WordPress scans the MU directory and loads the PHP file automatically. Do not wrap the code in an activation callback and expect that callback to fire: activation hooks do not run for MU plugins.
After uploading, load a page on the site and then open the WordPress dashboard. Go to Plugins and open the Must-Use tab. The file should be listed there. Seeing it in that tab confirms that WordPress recognized the file as an MU plugin; it does not mean that the plugin can be activated or deactivated from that screen.
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Install a multi-file MU plugin with a loader
WordPress loads PHP files that are directly inside mu-plugins. It does not automatically scan nested directories. For a plugin with classes, templates, and several source files, put the implementation in a subdirectory and add one top-level loader file.
For example, use this structure:
wp-content/
└── mu-plugins/
├── my-plugin-loader.php
└── my-plugin/
├── plugin.php
├── includes/
│ └── class-rules.php
└── assets/
└── settings.jsonThe top-level loader is the file WordPress sees. It can load the real plugin file with an absolute path based on the loader’s directory:
<?php
/**
* Plugin Name: Site Rules Loader
*/
require_once __DIR__ . '/my-plugin/plugin.php';Inside plugin.php, use paths relative to that file or define a base directory before requiring additional files. A loader prevents the common mistake of uploading only a subdirectory and then wondering why nothing appears in the Must-Use list. The loader itself must end in .php and must sit directly in mu-plugins.
Keep non-PHP assets, documentation, and tests in subdirectories. A PHP file left at the top level will be loaded on every request, even if it was intended only as a helper. If a helper is not meant to execute independently, keep it below the top level and require it from the loader.
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Top-level files only
WordPress loads PHP files in the first level of the MU directory. It does not recursively discover every PHP file below it. This is why a multi-file plugin needs a top-level loader.
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Alphabetical order
Files are loaded alphabetically. If one MU file must be initialized before another, teams commonly prefix filenames with numbers such as 00-bootstrap.php and 10-rules.php. Prefixes are a practical convention, not a documented requirement that guarantees a particular application architecture. Prefer explicit dependencies and a single loader when ordering is important.
Earlier than ordinary plugins and themes
MU plugins load before regular plugins and themes. That makes them suitable for policies or filters that must exist before another component initializes, but it also means a mistake can prevent later code from loading normally.
Every request
Every top-level PHP file is included on every WordPress request. Keep the code efficient, avoid expensive work that does not need to happen on every request, and use normal WordPress APIs rather than editing core files. A fatal error, syntax error, or incompatible function call can break the entire site rather than only one optional feature.
How to verify that an MU plugin works
- Check the remote path and confirm that the file is in
wp-content/mu-plugins, not inwp-content/pluginsor a nested directory without a loader. - Open the file and confirm it has valid PHP syntax and begins with
<?php. Do not add a closing PHP tag unless the project specifically requires one. - Visit the site once to make WordPress load the file.
- In the dashboard, open Plugins > Must-Use. Confirm the expected name and file are listed.
- Test the behavior the plugin is intended to provide, including an administrator request and a logged-out request if both paths matter.
- Review the PHP error log and the WordPress debug log for parse errors, missing files, undefined functions, or permission failures.
A plugin can appear in the Must-Use list and still contain logic that never runs because its hook is wrong, a condition is false, or a required file path is incorrect. The list verifies discovery, not the correctness of every code path.
How to disable or remove a must-use plugin
There is no Deactivate button for an MU plugin. Disable it from the server:
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- Connect with SFTP, SSH, or the host’s file manager.
- Open
wp-content/mu-plugins, or the host-defined MU directory. - Rename the file so it no longer has a PHP extension, for example
my-plugin.php.disabled, or move it outside the MU directory. - Load the site and verify that the problem or behavior is gone.
- Delete the file only after you have preserved a backup or confirmed it is no longer needed.
Renaming is safer than deleting during troubleshooting because it gives you an immediate rollback. For a multi-file plugin, rename or move the top-level loader; leaving the loader active will continue to require the subfolder. If the loader itself is healthy but one implementation file is broken, restore the known-good file or temporarily move the entire plugin directory as well.
Recovering from a fatal error
If the site returns a blank page, a critical-error screen, or a server error immediately after installation, use SFTP, SSH, or the host file manager rather than the WordPress dashboard. Rename the suspected top-level MU file, then retry the front end and login page. If the site recovers, inspect the PHP log, correct the code, and upload a tested replacement. Do not repeatedly refresh a broken production site while experimenting; each request executes the faulty file again.
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Host-managed and custom MU directories
The standard path is the right starting point for a normal WordPress installation, but managed hosts can provide their own arrangement. Some platforms install a host-owned MU plugin, and some reserve a separate directory for customer code. WordPress VIP documentation, for example, describes a /client-mu-plugins directory at the root of the site’s code repository rather than the conventional wp-content/mu-plugins path. A host may also add its own MU plugin for caching, monitoring, or platform integration.
The location can be changed with the WPMU_PLUGIN_DIR constant in wp-config.php. Before creating a second directory, inspect the configuration and follow the host’s deployment documentation. A custom path takes precedence over the conventional one for that site; uploading to the wrong directory will make the file appear to be missing.
Some hosts restrict direct writes to production or deploy MU plugins from version control. In that situation, use the host’s documented repository, deployment command, or support workflow instead of forcing a manual upload. Do not delete a host-supplied MU plugin merely because you did not create it; first identify what service depends on it.
Troubleshooting: when an MU plugin is not showing
| Symptom | Likely cause | What to check |
|---|---|---|
| The Must-Use tab is empty | The directory does not exist, is in the wrong WordPress installation, or a custom path is configured | Confirm the site root, inspect wp-content, and check WPMU_PLUGIN_DIR and host instructions |
| The file exists but is not listed | It is nested below the first level, has the wrong extension, or permissions prevent reading it | Move the PHP file directly into the MU directory, check the filename, and verify ownership and read permissions |
| A multi-file plugin does not load | Only the subfolder was uploaded; no top-level loader requires the main file | Add a loader such as my-plugin-loader.php and use a correct require_once path |
| The site breaks after upload | PHP syntax error, missing include, incompatible code, or an unhandled runtime error | Rename the top-level file to disable it, read the PHP/WordPress log, and restore a known-good version |
| The plugin appears but expected behavior is absent | The code depends on an activation hook, runs on a different hook, or its condition is not met | Move initialization into a normal runtime hook, test the relevant request type, and inspect logs |
| Renaming one file changes nothing | A loader or duplicate copy is still active | Search the MU directory and configured custom path for another top-level PHP file or loader |
Operational practices for reliable MU plugins
- Use version control: store the loader and implementation files in the same repository as the site’s deployment code when possible.
- Make changes reversible: deploy a new filename or keep the previous release so a rename or rollback can disable the change quickly.
- Keep one clear entry point: a single loader reduces accidental execution of helper files and makes load order easier to understand.
- Document ownership: record who maintains the file, what it does, and which host or service depends on it. An MU plugin may be invisible to a developer who only checks ordinary plugins.
- Test before production: check front-end pages, login, administration, cron jobs, REST requests, and command-line tasks that the code affects.
- Avoid activation assumptions: perform migrations or one-time setup through an explicit, repeatable routine rather than an activation hook that will never fire.
- Review permissions: the web server must be able to read the files, but write access should be limited to the deployment or administrator account.
- Watch compatibility: because MU plugins load before other extensions, use defensive checks when calling functions supplied by another plugin and fail gracefully when that dependency is absent.
When an MU plugin is the right choice
Choose an MU plugin when the code must be present on every request, should not be switched off by an ordinary dashboard user, or belongs to the site’s platform configuration rather than to an optional feature. It is also useful when a deployment process should make the code available immediately without a separate activation click.
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Do not use an MU plugin simply to hide a poorly maintained plugin from the Plugins page. Automatic loading increases the impact of defects and makes emergency access depend on server-level file access. Treat every MU file as production infrastructure: review it, test it, track it, and keep a recovery path.
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