Bootstrap

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.

Signature

Example

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

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

plugins_loaded is the earliest point where every active plugin's functions and classes are guaranteed to exist — use it for cross-plugin checks, not for registering WordPress objects.

Signature

Example

add_action( 'plugins_loaded', function() {
    if ( class_exists( 'WooCommerce' ) ) {
        require_once __DIR__ . '/integrations/woocommerce.php';
    }
} );

Common Use Cases

• Check if another plugin/class exists
• Load plugin translations
• Bootstrap plugin dependencies
• Set up autoloaders

after_setup_theme is where themes declare support for core features — post thumbnails, menus, HTML5 markup — before WordPress finishes bootstrapping.

Signature

Example

add_action( 'after_setup_theme', function() {
    add_theme_support( 'post-thumbnails' );
    add_theme_support( 'title-tag' );
    register_nav_menus( [ 'primary' => 'Primary Menu' ] );
} );

Common Use Cases

• Declare add_theme_support() features
• Register nav menu locations
• Set content width
• Load theme text domain

init is the default place to register post types, taxonomies, and rewrite-related APIs. It fires after plugins are loaded and before headers go out.

Signature

Example

add_action( 'init', function() {
    register_post_type( 'book', [
        'public' => true,
        'label'  => 'Books',
    ] );
} );

Common Use Cases

• Register custom post types
• Register custom taxonomies
• Start sessions
• Load text domains (plugins)

widgets_init is exclusively for register_sidebar() and register_widget() calls — nothing else belongs here.

Signature

Example

add_action( 'widgets_init', function() {
    register_sidebar( [
        'name' => 'Footer Widgets',
        'id'   => 'footer-widgets',
        'before_widget' => '<div class="widget %2$s">',
        'after_widget'  => '</div>',
    ] );
} );

Common Use Cases

• Register sidebar/widget areas
• Register custom WP_Widget classes
• Unregister default widgets

wp_loaded runs after every init callback has finished — use it when your code depends on things other plugins register on init.

Signature

Example

add_action( 'wp_loaded', function() {
    if ( ! post_type_exists( 'book' ) ) {
        return;
    }
    flush_rewrite_rules();
} );

Common Use Cases

• Read config set by other plugins on init
• Conditional logic that depends on registered post types/taxonomies
• Late-stage bootstrap checks