1. Creating Extensible
Plugins for WordPress
Hristo Chakarov
WordUp! Conference, Sofia, 2013

2. Who am I?
● I'm in the web since 2002
○ professionally since 2007
● Currently working as a Senior JavaScript
developer @ Netclime Inc.
○ one product - SiteKreator (website builder)
http://sitekreator.com
● Love WordPress
○ 2 plugins on the official plugins page
○ several commercial projects on top of the platform
○ co-founder of WordPress Bulgarian User Group
(WPBGUG)
http://wpbgug.org/

3. Why WordPress?
● Which is the best CMS in the World?
○ In fact, there's no such
● However, WordPress is much, much better
than its most famous competitors
○ Great Admin UI
○ Rich & well documented API
○ Easy to extend
■ you don't have to be very experienced
programmer in order to create a plugin
○ Huge community
■ tons of free & paid Themes & Plugins
○ But last 2 might be problematic

4. Today:
1. Why do we need to make plugins extensible
2. Anatomy of WordPress hooks
3. Differences between actions & filters
+ examples
4. Overwriting OOP-style plugins
5. Tips
6. Demo, plugin examples, good to read

5. Quick Questions
● How many of you have contributed at least 1
plugin in WordPress' Plugin Directory?
● How often do you feel the need to tweak a
3rd party plugin in order to fit 100% in your
project(s)?
● When you start coding new plugin, have you
ever felt the need to reuse functionality from
another your plugin?

6. Consider the following scenario:
1. You need extra functionality for your WP
project
2. You enjoy a plugin and you download it
3. There's that very tiny thing that this plugin
does not bring to you
4. You modify plugin's source code
5. You forget about modifying the source
6. A new version of the plugin is available. You
update your copy and after the update you
look at your site and...

8. ...Damn! My changes got lost!
Shooting at the developer won't solve the problem

9. What can be done?
Nothing.
But at least we can start making our own
plugins extensible and make the World better :)

10. Benefits of extensible plugin
● other developers can easily extend your
plugins so they serve their needs
● code reuse - we may have a base plugin
(core) and a lot of extensions (other plugins)
built on top of it

11. OK, but how do we make our WP
plugin extensible?
How do you extend WordPress?
● by using the platform's hooks
add_action
remove_action
add_filter
remove_filter
It's the same if you want to make your plugin
extensible - just register your own hooks!
● do_action
● apply_filters

12. How actions & filters work
(Minimum Theory)
● add_action & add_filter register handlers
(functions) to be executed on certain event
( hook_id, fn_name, priority, num_args )
● do_action & apply_filters execute all
handlers when called

13. Difference between Actions & Filters
● filters accept, modify & return data
○ strings
○ arrays
○ objects
● actions just execute
○ actions are just like events - they "trigger"

14. ● to modify strings (can be HTML or any other)
// in your core plugin
echo apply_filters(
'my_plugin_html',
'<strong>Hello, world!</strong>'
);
// in extending plugin
add_filter( 'my_plugin_html',
'custom_html_filter' );
function custom_html_filter( $html ) {
return '<div>' . $html . '</div>';
}
Use filters:

15. Use filters:
● for HTML its better to work on DOM level
○ Simple HTML DOM library is good for that
$dom = str_get_html('<b>Hello, world!</b>');
apply_filters( 'my_plugin_dom', $dom );
echo $dom->save();
add_filter( 'my_plugin_dom',
'custom_dom_filter' );
function custom_dom_filter( $dom ) {
// replace the <b> with <strong>
foreach ( $b as $dom->find('b') )
$b->outerhtml = '<strong>'.$b-
>innertext.'</strong>';
return $dom;

16. Use filters:
● to modify WP Query
// define initial set of query params
$query_args = array(
'post_type' => 'books',
'author' => 3
);
$books = new WP_Query( apply_filters(
'myplugin_query', $query_args
) );
add_filter('myplugin_query', 'modify_query');
function modify_query( $query_args ) {
$query_args['posts_per_page'] = 5;
return $query_args;

17. Use actions:
● to spice HTML markup

18. Use actions:
● to print resources
// in the core plugin
do_action( 'myplugin_print_resources' );
// in the extending plugin
add_action(
'myplugin_print_resources',
'myplugin_print_scripts'
);
function myplugin_print_scripts() {
echo '<script src=".."></script>
}

19. Lets make our plugin more OOP!
● define a main class for your plugin
● instantiate the class on WordPress <init>
class MyPlugin {
function myMethod() {
// ...
}
}
add_action( 'init', 'myplugin_init' );
function myplugin_init() {
global $plugin_instance;
$plugin_instance = new MyPlugin();
}

20. Overwrite
● define extending class & overwrite base
● instantiate the class on WordPress <init>
class ExtendingPlugin extends MyPlugin {
function myMethod() {
$this->parent(); // not necessary
// .. extra functionality here
}
}
remove_action( 'init', 'myplugin_init' );
add_action( 'init', 'extending_init' );
function extending_init() {
global $plugin_instance;
$plugin_instance = new ExtendingPlugin();
}

21. Tips
● plan your actions & filters carefully
○ think where you will need to provide a hook
○ try to use descriptive & easy to remember names
filter_html (too generic)
wpplg_tpl_html (what?)
a_really_awesome_filter_html (too long)
myplugin_filter_html (almost perfect)
○ don't overhook
● its good to create a documentation page
● comment, comment, comment!
○ comments can be your best friend

22. ● Image Widget
http://wordpress.org/extend/plugins/image-widget/
● NextGEN Gallery
http://wordpress.org/extend/plugins/nextgen-gallery/
● bbPress
http://wordpress.org/extend/plugins/bbpress/
● WPML
http://wpml.org/
● WooCommerce
http://www.woothemes.com/woocommerce/
Extensible plugins for inspiration

23. Extending NextGEN Gallery plugin
with custom template
Demo

24. I recommend you to read
● Anatomy of a WordPress Plugin
http://www.packtpub.com/article/anatomy-wordpress-plugin
● Inside WordPress Actions And Filters
http://wp.smashingmagazine.com/2012/02/16/inside-wordpress-
actions-filters/
● Writing Extensible Plugins With Actions and
Filters
http://wp.tutsplus.com/tutorials/plugins/writing-extensible-
plugins-with-actions-and-filters/
● 5 Things I’ve Learned About Writing Flexible
Plugins
http://www.kungfugrep.com/5-learned-writing-flexible-
plugins/

25. Time for questions
Your turn - word up!

26. mail (at) ickata.net
blog.ickata.net
facebook.com/ickatanet
github.com/ickata
Thank You!