Overview

WordPress follows a carefully orchestrated loading sequence that initializes the environment, loads core functionality, and prepares the system for request handling. This document outlines the complete bootstrap process.

Loading Sequence

1. Initial Entry Point

Request hits index.php or wp-admin/index.php:

index.php
  └── wp-blog-header.php
        ├── wp-load.php (loads WordPress)
        └── template-loader.php (loads theme template)

2. wp-load.php Sequence

wp-load.php
  ├── Locate wp-config.php
  └── Load wp-config.php
        └── wp-settings.php (main bootstrap)

3. wp-settings.php Bootstrap Order

This is the heart of WordPress initialization:

wp-settings.php
  │
  ├── [1] version.php - Version information
  │     └── Sets $wp_version, $wp_db_version, $required_php_version
  │
  ├── [2] load.php - Core loading functions
  │     └── Defines wp_fix_server_vars(), require_wp_db(), etc.
  │
  ├── [3] wp_check_php_mysql_versions()
  │     └── Validates PHP/MySQL requirements
  │
  ├── [4] default-constants.php
  │     └── wp_initial_constants() - Memory, debug, time constants
  │
  ├── [5] wp_fix_server_vars()
  │     └── Normalizes $_SERVER for IIS, CGI, etc.
  │
  ├── [6] wp_maintenance()
  │     └── Checks .maintenance file
  │
  ├── [7] timer_start()
  │     └── Starts page load timer
  │
  ├── [8] wp_debug_mode()
  │     └── Configures error reporting
  │
  ├── [9] wp_set_lang_dir()
  │     └── Sets WP_LANG_DIR constant
  │
  ├── [10] require_wp_db()
  │      └── Loads wpdb class, connects to database
  │
  ├── [11] wp_set_wpdb_vars()
  │      └── Configures table prefix, field types
  │
  ├── [12] wp_start_object_cache()
  │      └── Initializes object cache (or drop-in)
  │
  ├── [13] wp_plugin_directory_constants()
  │      └── Sets WP_PLUGIN_DIR, WPMU_PLUGIN_DIR
  │
  ├── [14] Load core includes
  │      └── compat.php, functions.php, class-wp.php, etc.
  │
  ├── [15] wp_not_installed()
  │      └── Redirects to installer if needed
  │
  ├── [16] wp_magic_quotes()
  │      └── Adds slashes to superglobals
  │
  ├── [17] Load mu-plugins
  │      └── wp_get_mu_plugins()
  │
  ├── [18] do_action('muplugins_loaded')
  │
  ├── [19] ms_cookie_constants() [multisite]
  │
  ├── [20] wp_cookie_constants()
  │
  ├── [21] wp_ssl_constants()
  │
  ├── [22] Load active plugins
  │      └── wp_get_active_and_valid_plugins()
  │
  ├── [23] do_action('plugins_loaded')
  │
  ├── [24] wp_functionality_constants()
  │
  ├── [25] do_action('setup_theme')
  │
  ├── [26] Load theme functions.php
  │
  ├── [27] do_action('after_setup_theme')
  │
  ├── [28] Register default widgets
  │
  ├── [29] do_action('init')
  │
  ├── [30] wp_templating_constants()
  │      └── TEMPLATEPATH, STYLESHEETPATH
  │
  └── [31] do_action('wp_loaded')

4. Request Processing (class-wp.php)

After wp_loaded, the WP class handles the request:

$wp->main()
  ├── init() - Set up current user
  ├── parse_request() - Parse URL, match rewrite rules
  ├── query_posts() - Execute main WP_Query
  ├── handle_404() - Set 404 status if needed
  ├── register_globals() - Export query vars to global scope
  ├── send_headers() - Send HTTP headers
  └── do_action('wp') - Request ready

5. Template Loading

After wp action fires, template is loaded:

template-loader.php
  ├── do_action('template_redirect')
  ├── Determine template (single.php, page.php, etc.)
  └── Include template file

Key Files

Bootstrap Hooks (Execution Order)

1. muplugins_loaded – After mu-plugins load
2. plugins_loaded – After plugins load
3. setup_theme – Before theme loads
4. after_setup_theme – After theme functions.php
5. init – WordPress fully initialized
6. wp_loaded – After all loading complete
7. wp – Request parsed and ready
8. template_redirect – Before template loads
9. shutdown – PHP shutdown

Short-Circuit Loading

SHORTINIT Mode

Define SHORTINIT = true before loading wp-settings.php for minimal bootstrap:

• Loads: version.php, load.php, compat.php, wpdb
• Skips: plugins, themes, most functions

Recovery Mode

When fatal errors occur, WordPress enters recovery mode:

• Pauses problematic plugins/themes
• Protected endpoints: wp-admin, wp-login.php
• Allows administrators to fix issues

Database Connection

// require_wp_db() in load.php
global $wpdb;
require_once ABSPATH . WPINC . '/class-wpdb.php';

// Check for db.php drop-in
if ( file_exists( WP_CONTENT_DIR . '/db.php' ) ) {
    require_once WP_CONTENT_DIR . '/db.php';
}

// Instantiate wpdb if not already done
$wpdb = new wpdb( DB_USER, DB_PASSWORD, DB_NAME, DB_HOST );

Object Cache Initialization

// wp_start_object_cache() in load.php

// Check for object-cache.php drop-in
if ( file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
    require_once WP_CONTENT_DIR . '/object-cache.php';
    wp_using_ext_object_cache( true );
}

// Initialize cache
wp_cache_init();

// Register global cache groups (for multisite)
wp_cache_add_global_groups( array( 'users', 'sites', ... ) );

Environment Detection

WordPress detects the environment type:

wp_get_environment_type()
// Returns: 'local', 'development', 'staging', or 'production'

// Set via:
// 1. Environment variable: WP_ENVIRONMENT_TYPE
// 2. Constant in wp-config.php: define('WP_ENVIRONMENT_TYPE', 'development')

Development Mode

wp_get_development_mode()
// Returns: 'core', 'plugin', 'theme', 'all', or ''

wp_is_development_mode( 'theme' )
// Check if specific development mode is active

Maintenance Mode

WordPress checks for .maintenance file in ABSPATH:

// .maintenance file content
<?php $upgrading = time(); ?>

// If file exists and is < 10 minutes old, maintenance mode is active