This document discusses WordPress plugins and database tables. It covers hooks, which allow plugins to connect functions to events. Hooks are either actions or filters. Database tables in WordPress use a metadata structure, with meta tables containing key-value pairs that provide flexibility. Taxonomies classify terms and allow terms to be associated with posts through term relationships and term taxonomies. Next week's topics will include adding admin menus and handling POST data for plugins.

1. WordPress Plugin #2
May 14th, 2015
Changwoo

2. Today’s Topics
1.Hooks in Nutshell
2.Database Tables in Nutshell

3. Recap
- Your first plugin
- Hello Dolly plugin
- action, and callback

4. Recap
Plugin Header:
/*
Plugin Name: Hello Dolly
Plugin URI: http://wordpress.org/plugins/hello-dolly/
Description: This is not just a plugin, it symbolizes the hope and enthusiasm of an
entire generation summed up in two words sung most famously by Louis Armstrong: Hello,
Dolly. When activated you will randomly see a lyric from <cite>Hello, Dolly</cite> in
the upper right of your admin screen on every page.
Author: Matt Mullenweg
Version: 1.6
Author URI: http://ma.tt/
*/

5. Recap
Callback Mechanism:
add_action( ‘admin_notices’, ‘hello_dolly’ );
function hello_dolly() {
...
}
Name of hook
Callback function
WP function for hook

6. Recap
Planning your own plugin, and ...
submission!

7. Hooks in Nutshell

8. Hook: an Event
Plugin programming:
- Hook-driven programming.
- A hook means an event.

9. Hello dolly’s case
2 actions: admin_notices, admin_head
● admin_notices codex
● admin_head codex

10. Hello dolly’s case
2 actions: admin_notices, admin_head
Let’s find source code.
$ fgrep -Rn do_action( 'admin_notices' ) *
wp-admin/admin-header.php:238: do_action( 'admin_notices' );
$ fgrep -Rn do_action( 'admin_head' ) *
wp-admin/includes/media.php:491: do_action( 'admin_head' );
wp-admin/includes/template.php:1627:do_action( 'admin_head' );
wp-admin/includes/class-wp-press-this.php:1267: do_action( 'admin_head' );
wp-admin/admin-header.php:125:do_action( 'admin_head' );
show source codes

11. Hook interface
wp-includes/plugin.php
What is action?
What is filter?
● add_filter()
● has_filter()
● apply_filters()
● add_action()
● has_action()
● do_action()

12. Task Separation
Themes: visual part
Plugins: functional part
Actions: structural part
Filters: information part

13. Task Separation
Q. I want to add extra contents to every post to be
displayed. Which one do I have to use?
Q. I want to a add menu page to admin page. Which one?
Q. I want to customize the table columns of pages screen.
Q. I want to do something when a post is updated.

14. Task Separation (solution)
Q. I want to add extra contents to every post to be
displayed. Which one do I have to use?
Q. I want to a add menu page to admin page. Which one?
Q. I want to customize the table columns of pages screen.
Q. I want to do something when a post is updated.
A. filter. ‘the_content’ hook.
A. action. ‘admin_menu’ hook.
A. filter. ‘manage_pages_columns’ hook.
A. action. ‘post_updated’ hook.

15. Task separation
Do not have to remember them all.
Moreover, actually,
add_action == add_filter

16. Action, Filter definition
wp-includes/plugin.php
function add_action($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
return add_filter($tag, $function_to_add, $priority, $accepted_args);
}
add_action() is mere an alias of add_filter()
Then, why add_action() is created?
Because they want to get more detailed code.
They just wanted detailed conception. That’s cool.

17. do_action, add_action feedback
do_action:
fires registered action
add_action:
loads any actions
No restriction. you can define your own
hooks.

18. callback functions...
....
..
.
$wp_filter
add_action(
$tag,
$callback,
$priority,
$num_args
);

19. callback functions...
....
..
.
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);

20. callback functions...
....
..
.
my_hook - my_callback
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);

21. callback functions...
....
..
.
my_hook - my_callback
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);
somewhere in your code
...
do_action(
‘my_hook’
);
...

22. callback functions...
....
..
.
my_hook - my_callback
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);
somewhere in your code
...
do_action(
‘my_hook’
);
...

23. callback functions...
....
..
.
my_hook - my_callback
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);
somewhere in your code
...
do_action(
‘my_hook’
);
...
by my_hook:
my_callback()

24. do_action
inside of do_action:
● retrieve the callback functions by their tags (hook)
● sort by priority
● call_user_func()
do {
foreach ( (array) current($wp_filter[$tag]) as $the_ )
if ( !is_null($the_['function']) )
call_user_func_array($the_['function'],
array_slice($args, 0, (int) $the_['accepted_args']));
} while ( next($wp_filter[$tag]) !== false );

25. add_action
function add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
global $wp_filter, $merged_filters;
$idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
$wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add,
'accepted_args' => $accepted_args);
unset( $merged_filters[ $tag ] );
return true;
}
$wp_filter is an array.

26. Check it out!
Build a plugin that dumps all registered
actions or filters.
Add your own hook and call it.
Download template code and edit.
http://<myip>/wp-admin/ (meetup / 1)
ftp://<myip> (meetup / 1)

27. Summary
● filter and action are the same. action is alias.
o structure, data separation
● In fact, this is like a fine wrapper for call_user_func.
o do_action() fires callback functions when the hook
is available
o add_action() loads callback functions.
● There are huge amount of hooks: see reference.
● You can also define your own hooks.

28. Database Tables in Nutshell

29. Database: the Root
Database: backbone of web apps.
A good app without understanding
its db structure: just an illusion.

30. Database description
https://codex.wordpress.org/Database_Description
● parent - meta styles
o wp_comments - wp_commentmeta
o wp_user - wp_usermeta
o wp_posts - wp_postmeta
o wp_options
● wp_terms - wp_term_taxonomy - wp_term_relationships

31. Meta table
Every *meta table has
● meta_key
● meta_value
closely related to “hashing”

32. Commentmeta structure

33. Postmeta structure

34. Usermeta structure

35. Options structure

36. Hashing
key - value pair.
key is short / value is often long.
key is searchable.
Hashing often called (implemented) as
“dictionary”, or “associative array”.

37. Meta key: benefits
Flexible, extendable
Ordinary tables can’t do these:
● Creating fields “on demands”
● This is one reason why WordPress is
a very versaitle, general, and
easy-to-customize CMS.

38. Meta key sample: user info
You want to add “Kakaotalk ID”
for every user:
just add “kakaotalk_id” meta key.

39. Meta in edit screen

40. Does it matter?
Yes! When fresh install,
WordPress: 11+ tables
74 tables
49 tables
79 tables
Jomla:
Drupal:
XpressEngine:

41. Meta key: Hazards
No type checking, validation
Meta key consistency
Efficiency
Nothing is perfect!
next: terms, taxonomies, ...

42. Terms and Taxonomies
Terms: a word
Taxonomy: classification of terms

43. Terms and Taxonomies
iron
??
Home appliance Metal ???

44. Adding Terms in Posts
Posts have basically two taxonomies:
● tags
● categories
Adding tag/category = adding term

45. Adding Terms in Posts
<terms>
term_id: 2
name: MyCategory
slug: my-category
term_group: 0

46. Adding Terms in Posts
<term_taxonomy>
term_taxonomy_id: 2
term_id: 2
taxonomy: category
/ post_tag
description:
parent: 0
count: 0

47. Category Setting
<term_relationships>
object_id: 1
term_taxonomy_id: 2
term_order: 0

48. Post ID: 75 term_relationships.object_id
term_relationships.term_taxonomy_id

49. Post ID: 75 term_relationships.object_id
term_relationships.term_taxonomy_id

50. Post ID: 75 term_relationships.object_id
term_relationships.term_taxonomy_id
term_taxonomy.term_taxonomy_id
term_taxonomy.term_id
term_taxonomy.taxonomy ★

51. Post ID: 75 term_relationships.object_id
term_relationships.term_taxonomy_id
term_taxonomy.term_taxonomy_id
term_taxonomy.term_id
term_taxonomy.taxonomy ★

52. Post ID: 75 term_relationships.object_id
term_relationships.term_taxonomy_id
terms.term_id
terms.name ★
terms.slug
term_taxonomy.term_taxonomy_id
term_taxonomy.term_id
term_taxonomy.taxonomy ★

53. Post ID: 75 term_relationships.object_id
term_relationships.term_taxonomy_id
terms.term_id
terms.name ★
terms.slug
term_taxonomy.term_taxonomy_id
term_taxonomy.term_id
term_taxonomy.taxonomy ★

54. Initial Taxonomies
< wp-includes/taxonomy.php >
function register_taxonomy(
$taxonomy, $object_type, $args = array() ) { ... }
function create_initial_taxonomies() {
register_taxonomy( 'category', 'post', array(...) );
register_taxonomy( 'post_tag', 'post', array(...) );
... and so on ...
create_initial_taxonomies() is called in wp-settings.php

55. Summary
● Wordpress Datbase table uses metadata
style strategy
● meta tables have key / value fields
o flexible, easy to extend
o while it can have some disadvantages

56. Summary
● Taxonomy: classification of term
● You can add taxonomies (as well as terms)
● Taxonomy can be flat or hierarchical
● term_taxonomy: taxonomy -
term
● term_relationship: posts - taxonomy

57. Next week
Admin Menu
● Almost every plugin has its menu
● Easy WP Plugin API
Admin Post
● POST type data submission
URL Query
AJAX Data handling
Wordpress Posts in Nutshell