Action Hook since 2.8.0

muplugins_loaded

Fires right after mu-plugins load — the true first hook of the request

Description

muplugins_loaded is the earliest action in the entire WordPress bootstrap — it fires right after must-use (mu-plugins) and network-activated plugins are loaded, before a single regular plugin has run. Almost nobody hooks it, which is exactly why it makes a great trivia question.

When it runs

Inside wp-settings.php, immediately after requiring files from wp-content/mu-plugins/ (and network-activated plugins on multisite), before regular active plugins are loaded.

Signature

do_action( 'muplugins_loaded' );

Examples

Basic

add_action( 'muplugins_loaded', function() {
    error_log( 'Bootstrap started.' );
} );

Confirm this is the earliest point custom code can hook into.

Real case

add_action( 'muplugins_loaded', function() {
    if ( ! defined( 'MY_PLUGIN_BOOT_TIME' ) ) {
        define( 'MY_PLUGIN_BOOT_TIME', microtime( true ) );
    }
} );

Mark a precise bootstrap-start timestamp for later performance measurement, before anything else has run.

Edge case

add_action( 'muplugins_loaded', function() {
    // Regular plugins are NOT loaded yet — calling a function
    // defined by a normal plugin here will fatal-error.
} );

Nothing from a normal (non-mu) plugin exists yet at this point — only mu-plugins and WordPress core itself.

Common Use Cases

Bootstrap code that must run before any regular plugin
Define constants needed by plugins loaded afterward
Multisite: react to network-activated plugins having loaded
Mark a precise start-of-bootstrap timestamp for performance profiling

Common mistakes

Assuming this hook only fires when mu-plugins exist — it fires on every request regardless, empty mu-plugins folder or not
Calling a function defined by a regular plugin from inside a muplugins_loaded callback — regular plugins have not loaded yet, that causes a fatal error

Related hooks

Fires before plugins_loaded

Articles

The Window of Opportunity: When Each WordPress Bootstrap Hook Is Open, and When It’s Already Closed

Both examples are correct code in the wrong hook. Why This Is Dangerous This isn’t always a reproducible bug. Whether it fires or not often depends on plugin…

muplugins_loaded: The Hook That Actually Fires First in WordPress Slug: muplugins-loaded-the-real-first-hook

Quick interview trap: which WordPress hook fires first? Ask ten WordPress developers “which hook runs first?” and nine will say init. The sharper ones say plugins_loaded. Almost nobody…

FAQ

Does muplugins_loaded fire if I have no mu-plugins at all?

Yes — WordPress calls this hook unconditionally on every request, whether the mu-plugins folder has files in it or not.

Can I use functions from my regular plugins inside a muplugins_loaded callback?

No — regular (non-mu) plugins have not loaded yet at this point. That is exactly what plugins_loaded is for.

Source: wp-settings.php

Group wizard

One session: this hook, then each related_hooks entry. Walk it and tell us what to change.

muplugins_loaded → plugins_loaded

functions.php — muplugins_loaded

functions.php

<?php
/**
 * Practice: hook into muplugins_loaded
 */
add_action( 'muplugins_loaded', function() {
	// Write your callback here.

} );

Group tour

muplugins_loaded · Hook 1 of 2 · Step 1 of 6

Meet muplugins_loaded

Fires right after mu-plugins load — the true first hook of the request

type actionphase bootstrapsince 2.8.0source wp-settings.php

do_action( 'muplugins_loaded' );

add_action( 'muplugins_loaded', function() {
    error_log( 'Bootstrap started.' );
} );

PHP muplugins_loaded · Hook 1 of 2 · Step 1 of 6

Practice the muplugins_loaded WordPress action hook in our interactive sandbox — a VS Code-style editor with a step-by-step muplugins_loaded tutorial, add_action exercises, and instant feedback. No local WordPress install required.