Behind the scenes Demystifying The black box of WordPress Core 

Evan Solomon Growth Engineer @ Automattic 

“Core” n. The hard or fibrous central part of certain fruits, such as the apple or pear, containing the seeds. 

“Core” n. The hard or fibrous central part of certain fruits, such as the apple or pear, containing the seeds. n. Those files you can download from WordPress.org. 

Don’t hack core != Don’t read core 

Reading Core != Here Be Dragons 

If you are reading this document in earnest, you should be comfortable reading documentation in code. So I won’t bother to copy and paste it here where it has little hope of staying current. ANDY SKELTON, AUTOMATTIC FIELD GUIDE 

1. Startup 

2. Files 

3. Tools 

4. Hooks 

5. Use it 

Startup /** * 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 */ 

index.php Everything starts here 

index.php Everything starts here wp-blog-header.php Load WP environment 

index.php Everything starts here wp-blog-header.php Load WP environment wp-load.php WordPress bootstrap 

index.php Everything starts here wp-blog-header.php Load WP environment wp-load.php WordPress bootstrap wp-settings.php Load /wp-includes 

index.php Everything starts here wp-blog-header.php Load WP environment wp-load.php WordPress bootstrap wp-settings.php Load /wp-includes Parse the request 

index.php Everything starts here wp-blog-header.php Load WP environment wp-load.php WordPress bootstrap wp-settings.php Load /wp-includes Parse the request template-loader.php Load theme template 

It’s impossible to know what WordPress is really doing without understanding this 

File structure -rw-r--r-- index.php -rw-r--r-- license.txt -rw-r--r-- readme.html -rw-r--r-- wp-activate.php drwxr-xr-x wp-admin -rw-r--r-- wp-app.php -rw-r--r-- wp-blog-header.php -rw-r--r-- wp-comments-post.php -rw-r--r-- wp-config-sample.php drwxr-xr-x wp-content -rw-r--r-- wp-cron.php drwxr-xr-x wp-includes -rw-r--r-- wp-links-opml.php -rw-r--r-- wp-load.php -rw-r--r-- wp-login.php -rw-r--r-- wp-mail.php -rw-r--r-- wp-settings.php -rw-r--r-- wp-signup.php -rw-r--r-- wp-trackback.php -rw-r--r-- xmlrpc.php $ ls -l | awk '{print $1"\t"$9}' 

File structure -rw-r--r-- index.php -rw-r--r-- license.txt -rw-r--r-- readme.html -rw-r--r-- wp-activate.php drwxr-xr-x wp-admin -rw-r--r-- wp-app.php -rw-r--r-- wp-blog-header.php -rw-r--r-- wp-comments-post.php -rw-r--r-- wp-config-sample.php drwxr-xr-x wp-content -rw-r--r-- wp-cron.php drwxr-xr-x wp-includes -rw-r--r-- wp-links-opml.php -rw-r--r-- wp-load.php -rw-r--r-- wp-login.php -rw-r--r-- wp-mail.php -rw-r--r-- wp-settings.php -rw-r--r-- wp-signup.php -rw-r--r-- wp-trackback.php -rw-r--r-- xmlrpc.php $ ls -l | awk '{print $1"\t"$9}' 

/wp-admin Mostly page/file relationships that make the wp-admin UI /wp-includes Mostly discrete functionality not meant to be accessed directly 

$ ls wp-includes SimplePie Text admin-bar.php atomlib.php author-template.php bookmark-template.php bookmark.php cache.php canonical.php capabilities.php category-template.php category.php class-feed.php class-http.php class-IXR.php class-json.php class-oembed.php class-phpass.php class-phpmailer.php class-pop3.php class-simplepie.php class-smtp.php class-snoopy.php class-wp-admin-bar.php class-wp-ajax-response.php class-wp-customize- control.php class-wp-customize-manager.php class-wp-customize-section.php class-wp-customize-setting.php class-wp-editor.php class-wp-embed.php class-wp-error.php class-wp-http-ixr-client.php class-wp-theme.php class-wp-walker.php class-wp-xmlrpc-server.php class-wp.php class.wp-dependencies.php class.wp-scripts.php class.wp-styles.php comment-template.php comment.php compat.php cron.php css default-constants.php default-filters.php default-widgets.php deprecated.php feed-atom-comments.php feed-atom.php feed-rdf.php feed-rss.php feed-rss2-comments.php feed-rss2.php feed.php formatting.php functions.php functions.wp-scripts.php functions.wp-styles.php general-template.php http.php images js kses.php l10n.php link-template.php load.php locale.php media.php meta.php ms-blogs.php ms-default-constants.php ms-default-filters.php ms-deprecated.php ms-files.php ms-functions.php ms-load.php ms-settings.php nav-menu-template.php nav-menu.php option.php pluggable-deprecated.php pluggable.php plugin.php pomo post-template.php post-thumbnail-template.php post.php query.php registration-functions.php registration.php rewrite.php rss-functions.php rss.php script-loader.php shortcodes.php taxonomy.php template-loader.php template.php theme-compat theme.php update.php user.php vars.php version.php widgets.php wlwmanifest.xml wp-db.php wp-diff.php 

Help! We need tools 

GUI & CLI Tools 

Debug Bar And friends 

Debug Bar Console 

Debug Bar Extender 

Debug Bar Cron 

die(); var_dump(); error_log(); 

something(); // Make sure this code actually runs die(); thing_i_care_about(); 

$ ack betterthangrep.com 

I had to invent integral and differential calculus to figure it out. ISSAC NEWTON, ON ELLIPTICAL ORBITS 

$ fx $ action $ filter gist.github.com/3568555 

$ fx get_user_by Magically opens the function definition wp-inclues/pluggable.php:135 

A quick review of actions & filters 

action An alert that something just happened or is about to happen filter A similar alert, with the added responsibility to manipulate data 

function es_alert_me() { send_the_alert( 'evan' ); } add_action( 'sound_the_alarm', 'es_alert_me' ); /** * Some other code runs... */ do_action( 'sound_the_alarm' ); // es_alert_me() runs add_action() 

// Always turn sirens to bells function es_set_alert( $alert ) { return ( 'bell' == $alert ) ? 'siren' : $alert; } add_filter( 'alert_type', 'es_set_alert' ); /** * Some other code runs... */ $alert = 'bell'; $alert = apply_filters( 'alert_type', $alert ); // Now $alert is 'siren' add_filter() 

No content

function add_action( /* arguments */ ) { return add_filter( /* same arguments */ ); } wp-includes/plugin.php 

Use it! 

Links are boring 

I like oEmbed, I wish it worked in my comments, too. ME, SOME TIME IN AUGUST 

Maybe comments get filtered. $ ack apply_filters wp-includes/ | ack comment 

Maybe comments get filtered. $ ack apply_filters wp-includes/ | ack comment TOO. Many. Results. 140! 

How does TwentyEleven use comments? 

Where does that come from? $ ack comment-content wp-content/ themes/twentyeleven --php wp-content/themes/twentyeleven/functions.php 562: 

Where does that come from? $ ack comment-content wp-content/ themes/twentyeleven --php wp-content/themes/twentyeleven/functions.php 562: 

This function looks promising $ fx comment_text function comment_text( $comment_ID = 0 ) { $comment = get_comment( $comment_ID ); echo apply_filters( 'comment_text', get_comment_text( $comment_ID ), $comment ); } 

This function looks promising $ fx comment_text function comment_text( $comment_ID = 0 ) { $comment = get_comment( $comment_ID ); echo apply_filters( 'comment_text', get_comment_text( $comment_ID ), $comment ); } 

I bet this changes comments function es_comment_text( $comment_text ) { return $comment_text . 'I SHOULD SEE THIS'; } add_filter( 'comment_text', 'es_comment_text' ); 

I bet this changes comments function es_comment_text( $comment_text ) { return $comment_text . 'I SHOULD SEE THIS'; } add_filter( 'comment_text', 'es_comment_text' ); Fatal error! 

Let’s see why it breaks function es_comment_text( $comment_text ) { error_log( var_export( $comment_text, true ) ); return $comment_text; } add_filter( 'comment_text', 'es_comment_text' ); 

Let’s see why it breaks function es_comment_text( $comment_text ) { error_log( var_export( $comment_text, true ) ); return $comment_text; } add_filter( 'comment_text', 'es_comment_text' ); array ( 0 => 'comment text here...', ) 

Use the array function es_comment_text( $comment_text ) { $comment_text[ 0 ] += " I SHOULD SEE THIS"; return $comment_text; } add_filter( 'comment_text', 'es_comment_text' ); 

Use the array function es_comment_text( $comment_text ) { $comment_text[ 0 ] += " I SHOULD SEE THIS"; return $comment_text; } add_filter( 'comment_text', 'es_comment_text' ); It works! 

There’s probably an oEmbed filter $ ack add_filter wp-includes/ | ack oembed wp-includes/class-oembed.php:54: add_filter( 'oembed_dataparse', array(&$this, '_strip_newlines'), 10, 3 ); wp-includes/media.php:1058: add_filter( 'the_content', array(&$this, 'autoembed'), 8 ); 

Test the simplest usage first add_filter( 'comment_text', array( $wp_embed, 'autoembed' ) ); 

Test the simplest usage first add_filter( 'comment_text', array( $wp_embed, 'autoembed' ) ); Nope, just a link 

I know what makes automatic links $ ack add_filter wp-includes/ | ack comment_text | ack make_clickable wp-includes/default-filters.php: 151: add_filter( 'comment_text', 'make_clickable', 9 ); 

Try going earlier add_filter( 'comment_text', array( $wp_embed, 'autoembed' ), 8 ); It works! 

Videos are cool 

Finished Plugin wordpress.org/extend/plugins/ oembed-in-comments/ github.com/evansolomon/ wp-oembed-in-comments & 

class ES_oEmbed_Comments { function __construct() { add_action( 'init', array( $this, 'init' ) ); } function init() { if ( ! is_admin() ) $this->oembed_in_comments(); } function oembed_in_comments() { $clickable = has_filter( 'comment_text', 'make_clickable' ); $priority = ( $clickable ) ? $clickable - 1 : 10; add_filter( 'comment_text', array( $this, 'oembed_filter' ), $priority ); } function oembed_filter( $comment_text ) { global $wp_embed; add_filter( 'embed_oembed_discover', '__return_false', 999 ); $comment_text = $wp_embed->autoembed( $comment_text ); remove_filter( 'embed_oembed_discover', '__return_false', 999 ); return $comment_text; } } new ES_oEmbed_Comments; Finished Plugin