This article will describe the steps involved in the WordPress application flow. The initialisation steps shown below will show how WordPress is bootstrapped, how a database connection is established, how plugins are loaded and more.
As with many PHP applications, the process starts with the index.php file located in the WordPress document root.
<?php
/**
* Front to the WordPress application. This file doesn't do anything, but loads
* wp-blog-header.php which does and tells WordPress to load the theme.
*
* @package WordPress
*/
/**
* Tells WordPress to load the WordPress theme and output it.
*
* @var bool
*/
define('WP_USE_THEMES', true);
/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . '/wp-blog-header.php' );
Fortunately we’re blessed with useful comments in this file. A WP_USE_THEMES constant is set to true to load the theme and the rest of the execution involves the wp-blog-header.php file.
Within the same directory level, you should find wp-blog-header.php.
<?php
/**
* Loads the WordPress environment and template.
*
* @package WordPress
*/
if ( !isset($wp_did_header) ) {
$wp_did_header = true;
// Load the WordPress library.
require_once( dirname(__FILE__) . '/wp-load.php' );
// Set up the WordPress query.
wp();
// Load the theme template.
require_once( ABSPATH . WPINC . '/template-loader.php' );
}
The code above is wrapped within an isset() check to ensure it is only run once.
The WordPress library is first loaded within the wp-load.php file.
<?php
/**
* Bootstrap file for setting the ABSPATH constant
* and loading the wp-config.php file. The wp-config.php
* file will then load the wp-settings.php file, which
* will then set up the WordPress environment.
*
* If the wp-config.php file is not found then an error
* will be displayed asking the visitor to set up the
* wp-config.php file.
*
* Will also search for wp-config.php in WordPress' parent
* directory to allow the WordPress directory to remain
* untouched.
*
* @package WordPress
*/
/** Define ABSPATH as this file's directory */
if ( ! defined( 'ABSPATH' ) ) {
define( 'ABSPATH', dirname( __FILE__ ) . '/' );
}
error_reporting( E_CORE_ERROR | E_CORE_WARNING | E_COMPILE_ERROR | E_ERROR | E_WARNING | E_PARSE | E_USER_ERROR | E_USER_WARNING | E_RECOVERABLE_ERROR );
/*
* If wp-config.php exists in the WordPress root, or if it exists in the root and wp-settings.php
* doesn't, load wp-config.php. The secondary check for wp-settings.php has the added benefit
* of avoiding cases where the current directory is a nested installation, e.g. / is WordPress(a)
* and /blog/ is WordPress(b).
*
* If neither set of conditions is true, initiate loading the setup process.
*/
if ( file_exists( ABSPATH . 'wp-config.php') ) {
/** The config file resides in ABSPATH */
require_once( ABSPATH . 'wp-config.php' );
} elseif ( @file_exists( dirname( ABSPATH ) . '/wp-config.php' ) && ! @file_exists( dirname( ABSPATH ) . '/wp-settings.php' ) ) {
/** The config file resides one level above ABSPATH but is not part of another installation */
require_once( dirname( ABSPATH ) . '/wp-config.php' );
} else {
// A config file doesn't exist
define( 'WPINC', 'wp-includes' );
require_once( ABSPATH . WPINC . '/load.php' );
// Standardize $_SERVER variables across setups.
wp_fix_server_vars();
require_once( ABSPATH . WPINC . '/functions.php' );
$path = wp_guess_url() . '/wp-admin/setup-config.php';
/*
* We're going to redirect to setup-config.php. While this shouldn't result
* in an infinite loop, that's a silly thing to assume, don't you think? If
* we're traveling in circles, our last-ditch effort is "Need more help?"
*/
if ( false === strpos( $_SERVER['REQUEST_URI'], 'setup-config' ) ) {
header( 'Location: ' . $path );
exit;
}
define( 'WP_CONTENT_DIR', ABSPATH . 'wp-content' );
require_once( ABSPATH . WPINC . '/version.php' );
wp_check_php_mysql_versions();
wp_load_translations_early();
// Die with an error message
$die = sprintf(
/* translators: %s: wp-config.php */
__( "There doesn't seem to be a %s file. I need this before we can get started." ),
'<code>wp-config.php</code>'
) . '</p>';
$die .= '<p>' . sprintf(
/* translators: %s: Codex URL */
__( "Need more help? <a href='%s'>We got it</a>." ),
__( 'https://codex.wordpress.org/Editing_wp-config.php' )
) . '</p>';
$die .= '<p>' . sprintf(
/* translators: %s: wp-config.php */
__( "You can create a %s file through a web interface, but this doesn't work for all server setups. The safest way is to manually create the file." ),
'<code>wp-config.php</code>'
) . '</p>';
$die .= '<p><a href="' . $path . '" class="button button-large">' . __( "Create a Configuration File" ) . '</a>';
wp_die( $die, __( 'WordPress › Error' ) );
}
This file is responsible for a few things.
Firstly, the file tries to load the wp-config.php file where the WordPress database connection details are found. The file is usually located in the document root, but WordPress does allow you to move the file up a directory for security purposes. This is done via the use of the ABSPATH constant.
Assuming that database credentials have already been entered and WordPress has run through its installation process, head to the wp-config.php file.
Right at the bottom of the file will be an include to the wp-settings.php file.
/** Sets up WordPress vars and included files. */ require_once(ABSPATH . 'wp-settings.php');
The wp-settings.php file contains a lot of functionality, so it’ll be summarised as best as possible.
The wp-includes/load.php file is included, which contains many initialisation functions that are used later on in the bootstrapping process.
define( 'WPINC', 'wp-includes' ); // Include files required for initialization. require( ABSPATH . WPINC . '/load.php' );
wp-settings.php also includes the wp-includes/version.php file which contains the version of WordPress installed, as well as the required PHP and MySQL versions to use the current WordPress version.
/* * These can't be directly globalized in version.php. When updating, * we're including version.php from another installation and don't want * these values to be overridden if already set. */ global $wp_version, $wp_db_version, $tinymce_version, $required_php_version, $required_mysql_version, $wp_local_package; require( ABSPATH . WPINC . '/version.php' );
Further down, the actual check made against the PHP and MySQL versions the user is using is made using the wp_check_php_mysql_versions() function.
// Check for the required PHP version and for the MySQL extension or a database drop-in. wp_check_php_mysql_versions();
The wp-includes/functions.php is included which defines the main WordPress API. This is loaded along with other WordPress files.
// Load early WordPress files. require( ABSPATH . WPINC . '/compat.php' ); require( ABSPATH . WPINC . '/class-wp-list-util.php' ); require( ABSPATH . WPINC . '/functions.php' ); require( ABSPATH . WPINC . '/class-wp-matchesmapregex.php' ); require( ABSPATH . WPINC . '/class-wp.php' ); require( ABSPATH . WPINC . '/class-wp-error.php' ); require( ABSPATH . WPINC . '/pomo/mo.php' );
The require_wp_db() defined in wp-includes/load.php is then executed. This loads in wp-includes/wp-db.php which defines the wpdb class.
The $wpdb object is instantiated and a connection with the database is established.
// Include the wpdb class and, if present, a db.php database drop-in. global $wpdb; require_wp_db();
The default filters are included via the wp-includes/default-filters.php file. This file sets up the default filters and actions for most of the WordPress hooks.
// Attach the default filters. require( ABSPATH . WPINC . '/default-filters.php' );
The L10n libraries are then included. These provide useful functions such as enabling you to make use of WordPress translations.
// Load the L10n library. require_once( ABSPATH . WPINC . '/l10n.php' ); require_once( ABSPATH . WPINC . '/class-wp-locale.php' ); require_once( ABSPATH . WPINC . '/class-wp-locale-switcher.php' );
There are then a bunch of libraries that are included together.
// Load most of WordPress. require( ABSPATH . WPINC . '/class-wp-walker.php' ); require( ABSPATH . WPINC . '/class-wp-ajax-response.php' ); require( ABSPATH . WPINC . '/formatting.php' ); require( ABSPATH . WPINC . '/capabilities.php' ); require( ABSPATH . WPINC . '/class-wp-roles.php' ); require( ABSPATH . WPINC . '/class-wp-role.php' ); require( ABSPATH . WPINC . '/class-wp-user.php' ); require( ABSPATH . WPINC . '/class-wp-query.php' ); require( ABSPATH . WPINC . '/query.php' ); require( ABSPATH . WPINC . '/date.php' ); require( ABSPATH . WPINC . '/theme.php' ); require( ABSPATH . WPINC . '/class-wp-theme.php' ); require( ABSPATH . WPINC . '/template.php' ); require( ABSPATH . WPINC . '/user.php' ); require( ABSPATH . WPINC . '/class-wp-user-query.php' ); require( ABSPATH . WPINC . '/class-wp-session-tokens.php' ); require( ABSPATH . WPINC . '/class-wp-user-meta-session-tokens.php' ); require( ABSPATH . WPINC . '/meta.php' ); require( ABSPATH . WPINC . '/class-wp-meta-query.php' ); require( ABSPATH . WPINC . '/class-wp-metadata-lazyloader.php' ); require( ABSPATH . WPINC . '/general-template.php' ); require( ABSPATH . WPINC . '/link-template.php' ); require( ABSPATH . WPINC . '/author-template.php' ); require( ABSPATH . WPINC . '/post.php' ); require( ABSPATH . WPINC . '/class-walker-page.php' ); require( ABSPATH . WPINC . '/class-walker-page-dropdown.php' ); require( ABSPATH . WPINC . '/class-wp-post-type.php' ); require( ABSPATH . WPINC . '/class-wp-post.php' ); require( ABSPATH . WPINC . '/post-template.php' ); require( ABSPATH . WPINC . '/revision.php' ); require( ABSPATH . WPINC . '/post-formats.php' ); require( ABSPATH . WPINC . '/post-thumbnail-template.php' ); require( ABSPATH . WPINC . '/category.php' ); require( ABSPATH . WPINC . '/class-walker-category.php' ); require( ABSPATH . WPINC . '/class-walker-category-dropdown.php' ); require( ABSPATH . WPINC . '/category-template.php' ); require( ABSPATH . WPINC . '/comment.php' ); require( ABSPATH . WPINC . '/class-wp-comment.php' ); require( ABSPATH . WPINC . '/class-wp-comment-query.php' ); require( ABSPATH . WPINC . '/class-walker-comment.php' ); require( ABSPATH . WPINC . '/comment-template.php' ); require( ABSPATH . WPINC . '/rewrite.php' ); require( ABSPATH . WPINC . '/class-wp-rewrite.php' ); require( ABSPATH . WPINC . '/feed.php' ); require( ABSPATH . WPINC . '/bookmark.php' ); require( ABSPATH . WPINC . '/bookmark-template.php' ); require( ABSPATH . WPINC . '/kses.php' ); require( ABSPATH . WPINC . '/cron.php' ); require( ABSPATH . WPINC . '/deprecated.php' ); require( ABSPATH . WPINC . '/script-loader.php' ); require( ABSPATH . WPINC . '/taxonomy.php' ); require( ABSPATH . WPINC . '/class-wp-taxonomy.php' ); require( ABSPATH . WPINC . '/class-wp-term.php' ); require( ABSPATH . WPINC . '/class-wp-term-query.php' ); require( ABSPATH . WPINC . '/class-wp-tax-query.php' ); require( ABSPATH . WPINC . '/update.php' ); require( ABSPATH . WPINC . '/canonical.php' ); require( ABSPATH . WPINC . '/shortcodes.php' ); require( ABSPATH . WPINC . '/embed.php' ); require( ABSPATH . WPINC . '/class-wp-embed.php' ); require( ABSPATH . WPINC . '/class-oembed.php' ); require( ABSPATH . WPINC . '/class-wp-oembed-controller.php' ); require( ABSPATH . WPINC . '/media.php' ); require( ABSPATH . WPINC . '/http.php' ); require( ABSPATH . WPINC . '/class-http.php' ); require( ABSPATH . WPINC . '/class-wp-http-streams.php' ); require( ABSPATH . WPINC . '/class-wp-http-curl.php' ); require( ABSPATH . WPINC . '/class-wp-http-proxy.php' ); require( ABSPATH . WPINC . '/class-wp-http-cookie.php' ); require( ABSPATH . WPINC . '/class-wp-http-encoding.php' ); require( ABSPATH . WPINC . '/class-wp-http-response.php' ); require( ABSPATH . WPINC . '/class-wp-http-requests-response.php' ); require( ABSPATH . WPINC . '/class-wp-http-requests-hooks.php' ); require( ABSPATH . WPINC . '/widgets.php' ); require( ABSPATH . WPINC . '/class-wp-widget.php' ); require( ABSPATH . WPINC . '/class-wp-widget-factory.php' ); require( ABSPATH . WPINC . '/nav-menu.php' ); require( ABSPATH . WPINC . '/nav-menu-template.php' ); require( ABSPATH . WPINC . '/admin-bar.php' ); require( ABSPATH . WPINC . '/rest-api.php' ); require( ABSPATH . WPINC . '/rest-api/class-wp-rest-server.php' ); require( ABSPATH . WPINC . '/rest-api/class-wp-rest-response.php' ); require( ABSPATH . WPINC . '/rest-api/class-wp-rest-request.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-posts-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-attachments-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-post-types-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-post-statuses-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-revisions-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-taxonomies-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-terms-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-users-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-comments-controller.php' ); require( ABSPATH . WPINC . '/rest-api/endpoints/class-wp-rest-settings-controller.php' ); require( ABSPATH . WPINC . '/rest-api/fields/class-wp-rest-meta-fields.php' ); require( ABSPATH . WPINC . '/rest-api/fields/class-wp-rest-comment-meta-fields.php' ); require( ABSPATH . WPINC . '/rest-api/fields/class-wp-rest-post-meta-fields.php' ); require( ABSPATH . WPINC . '/rest-api/fields/class-wp-rest-term-meta-fields.php' ); require( ABSPATH . WPINC . '/rest-api/fields/class-wp-rest-user-meta-fields.php' );
Some of the more key libraries to look at can be seen below.
wp-includes/wp-query.php defines functions that are part of the WordPress Query API, and the WP_Query class is instantiated into the $wp_queryglobal object. This is used by functions like the_post().
The wp-includes/theme.php file defines lots of useful theme-related functions such as get_stylesheet_uri().
wp-includes/general-template.php defines general template functions like get_header() and get_footer().
The wp-includes/post.php file defines post functions like register_post_type(), get_post() and more.
wp-includes/rewrite.php involves the WP_Rewrite class which is the core class of the Rewrite API. The class handles request routing and permalinks.
The wp-includes/update.php file contains functions that assist with updating the WordPress core.
Moving further down wp-settings.php, the default theme directory is registered.
// Register the default theme directory root register_theme_directory( get_theme_root() );
The active plugins are then loaded. This is where any install plugins’ functionality gets loaded and executed.
// Load active plugins.
foreach ( wp_get_active_and_valid_plugins() as $plugin ) {
wp_register_plugin_realpath( $plugin );
include_once( $plugin );
}
unset( $plugin );
The functions.php file for any active themes are loaded from the child and parent themes.
// Load the functions for the active theme, for both parent and child theme if applicable.
if ( ! wp_installing() || 'wp-activate.php' === $pagenow ) {
if ( TEMPLATEPATH !== STYLESHEETPATH && file_exists( STYLESHEETPATH . '/functions.php' ) )
include( STYLESHEETPATH . '/functions.php' );
if ( file_exists( TEMPLATEPATH . '/functions.php' ) )
include( TEMPLATEPATH . '/functions.php' );
}
$GLOBALS['wp']->init() is executed which sets up the WordPress user.
// Set up current user. $GLOBALS['wp']->init();
And finally, the last steps within wp-settings.php involve running the init and wp-loaded actions.
/**
* Fires after WordPress has finished loading but before any headers are sent.
*
* Most of WP is loaded at this stage, and the user is authenticated. WP continues
* to load on the {@see 'init'} hook that follows (e.g. widgets), and many plugins instantiate
* themselves on it for all sorts of reasons (e.g. they need a user, a taxonomy, etc.).
*
* If you wish to plug an action once WP is loaded, use the {@see 'wp_loaded'} hook below.
*
* @since 1.5.0
*/
do_action( 'init' );
// Check site status
if ( is_multisite() ) {
if ( true !== ( $file = ms_site_check() ) ) {
require( $file );
die();
}
unset($file);
}
/**
* This hook is fired once WP, all plugins, and the theme are fully loaded and instantiated.
*
* Ajax requests should use wp-admin/admin-ajax.php. admin-ajax.php can handle requests for
* users not logged in.
*
* @link https://codex.wordpress.org/AJAX_in_Plugins
*
* @since 3.0.0
*/
do_action( 'wp_loaded' );
At long last, head back to the wp-blog-header.php file and look at the wp() line.
// Set up the WordPress query. wp();
This function resides within the wp-includes/functions.php file and sets up the WordPress query.
function wp( $query_vars = '' ) {
global $wp, $wp_query, $wp_the_query;
$wp->main( $query_vars );
if ( !isset($wp_the_query) )
$wp_the_query = $wp_query;
}
The final piece of code within wp-blog-header.php involves including the wp-includes/template-loader.php file.
// Load the theme template. require_once( ABSPATH . WPINC . '/template-loader.php' );
This file is responsible for loading the correct template based on the visitor’s URL. The functions make sure that the highest priority template is loaded.
<?php
/**
* @package WordPress
*/
if ( defined('WP_USE_THEMES') && WP_USE_THEMES )
/**
* Fires before determining which template to load.
*
* @since 1.5.0
*/
do_action( 'template_redirect' );
/**
* Filters whether to allow 'HEAD' requests to generate content.
*
* Provides a significant performance bump by exiting before the page
* content loads for 'HEAD' requests. See #14348.
*
* @since 3.5.0
*
* @param bool $exit Whether to exit without generating any content for 'HEAD' requests. Default true.
*/
if ( 'HEAD' === $_SERVER['REQUEST_METHOD'] && apply_filters( 'exit_on_http_head', true ) )
exit();
// Process feeds and trackbacks even if not using themes.
if ( is_robots() ) :
/**
* Fired when the template loader determines a robots.txt request.
*
* @since 2.1.0
*/
do_action( 'do_robots' );
return;
elseif ( is_feed() ) :
do_feed();
return;
elseif ( is_trackback() ) :
include( ABSPATH . 'wp-trackback.php' );
return;
endif;
if ( defined('WP_USE_THEMES') && WP_USE_THEMES ) :
$template = false;
if ( is_embed() && $template = get_embed_template() ) :
elseif ( is_404() && $template = get_404_template() ) :
elseif ( is_search() && $template = get_search_template() ) :
elseif ( is_front_page() && $template = get_front_page_template() ) :
elseif ( is_home() && $template = get_home_template() ) :
elseif ( is_post_type_archive() && $template = get_post_type_archive_template() ) :
elseif ( is_tax() && $template = get_taxonomy_template() ) :
elseif ( is_attachment() && $template = get_attachment_template() ) :
remove_filter('the_content', 'prepend_attachment');
elseif ( is_single() && $template = get_single_template() ) :
elseif ( is_page() && $template = get_page_template() ) :
elseif ( is_singular() && $template = get_singular_template() ) :
elseif ( is_category() && $template = get_category_template() ) :
elseif ( is_tag() && $template = get_tag_template() ) :
elseif ( is_author() && $template = get_author_template() ) :
elseif ( is_date() && $template = get_date_template() ) :
elseif ( is_archive() && $template = get_archive_template() ) :
else :
$template = get_index_template();
endif;
/**
* Filters the path of the current template before including it.
*
* @since 3.0.0
*
* @param string $template The path of the template to include.
*/
if ( $template = apply_filters( 'template_include', $template ) ) {
include( $template );
} elseif ( current_user_can( 'switch_themes' ) ) {
$theme = wp_get_theme();
if ( $theme->errors() ) {
wp_die( $theme->errors() );
}
}
return;
endif;
There is no more code to execute within the wp-blog-header.php or index.php files, and thus, the WordPress application flow process is complete.
Whilst not every little detail of the initialisation has been covered, you should have enough knowledge of the WordPress core to debug specific areas of the application.
Note: This article is based on WordPress version 4.9.4.