WordPress Plugin Development For Beginners

WordPress Plugin Development for Beginners John P. Bloch Senior Web Developer Avendi Media, Inc. http://www.johnpbloch.com/ @johnpbloch

What Are Plugins?
The stupid answer:
Anything you want*
*theoretically, at least
The useful answer:
Non-essential and elective aspects of any given WordPress installation

Wait a Minute...
Aren't plugins just extensions of WordPress? Don't they just add functionality that isn't there?

Plugins are Part of WordPress
Plugins have primacy of place
Plugins interact with WordPress in the same way WordPress interacts with itself

An Example Plugin

How Do Plugins Work?
Almost all plugin use hooks. There are two kinds of hooks:
Actions
Filters
What's the difference?
Filters expect you to return something, and...
… that's it!
Hooks let you tell WordPress to execute a function when the specified hook fires

How To Use Hooks
Filters:
<?php add_filter( 'filter_name', 'function_name' ); ?>
Actions:
<?php add_action( 'action_name', 'func_name' ); ?>
Both functions take two optional arguments:
Priority
Accepted Arguments

How To Fire Hooks
To fire an action:
<?php do_action( 'action_name' ); ?>
This fires all functions hooked into that action
To fire a filter:
<?php apply_filters( 'filter_name', 'Filter value' ); ?>
The 'Filter value' will be the first argument of each hooked function
apply_filters will return the filtered input

How Does Priority Work?
Actions and filters are executed in the order they are added. So this code will echo: First Second Third

How Does Priority Work?
Priority changes this order. Actions and filters have a default priority of 10. Using a lower number gives it higher priority. This code ouputs: Third First Second

Accepted Arguments
Accepted arguments allows you to specify how many arguments to send to a hooked function
Defaults to 1 for both actions and filters
The following would output: Don't mention Joomla Don't mention

Some Common Hooks
Actions
init
template_redirect
admin_init
after_setup_theme
wp_loaded
wp_head
wp_footer
admin_menu
Filters
the_content
the_title
wp_title
template_include
body_class
post_class

But Wait! There's More!
One more way to hook into WordPress is through shortcodes
Shortcodes are inserted into a post or page from the editor; WordPress replaces the shortcode tags with dynamically generated content
Shortcodes are kind of like HTML:
[foo bar='bar']
[foo bar='bar']Some Content[/foo]

How to Add a Shortcode
Shortcodes must be registered:
<?php add_shortcode( 'tag_name', 'function_name' ); ?>
Shortcode handlers take two optional arguments:
Arguments: an array of arguments from the shortcode
Content: the content (if any) of the shortcode
Shortcode handlers should return, not echo, their content

An Example of a Shortcode

Developer Toolbox
Local development environment
LAMPP Stack (XAMPP – http://apachefriends.org )
Subversion
Windows: TortoiseSVN or Cygwin
Everything else: CLI
Text Editor
FTP client
Mac: Transmit
SSH Client
Windows: Putty
Reference
PHPXref ( http://www.phpxref.com )

WordPress Developer Toolbox
Official Support Channels
WordPress Codex ( http://codex.wordpress.org/ )
WordPress Forums ( http://wordpress.org/support/ )
IRC Channel #wordpress (chat.freenode.net)
Mailing Lists (sort of) ( http://codex.wordpress.org/Mailing_Lists )
Unofficial Support Channels
WordPress Answers ( http://wordpress.stackexchange.com/ )
WordPress Docs ( http://wpdocs.labs.thedextrousweb.com/ )
Alot of Books

Just Because

Adding Your Plugin to the Directory
http://wordpress.org/extend/plugins/add/
Answer a few questions
Name – Very important! This will determine the name of the directory your plugin creates
Description
Plugin URL (optional)
… And wait!

What is SVN? SVN, or Subversion, is a version control program. Others include Git and Mercurial. Subversion is used for core WordPress development/distribution and official plugin development/distribution. Once you have your repository, anybody in the world can check it out. Only you* can check files into it. *Technically, Nacin and some others have global plugin repo commit access. #blamenacin

How to Use SVN
If you are on windows and use TortoiseSVN, just follow these next steps substituting ”Click X menu item” for ”Execute X command”
Go to the directory into which you'd like the repo to go:
~$ cd Documents/svn
Check out your repository for the first time:
~$ svn co http://plugins.svn.wordpress.org/custom-post-permalinks/ custom-post-permalinks
Edit to your heart's content

How to Use SVN
When you're done making changes (or reach a good stopping point), commit your changes:
~$ svn ci -m 'This is a description of the commit!'
… Tada!

Some Other Notes on SVN
Updating your plugin
Tagging vs. Stable Trunk
Branching
Adding committers (and other admin tools)

Further Reading
Core.
Core.
Core.
Seriously. Just read core's runtime execution.
Line by line.

Now Let's Get Down to Business! Time to switch gears, open VIM, and write a plugin!

WordPress Plugin Development For Beginners

by johnpbloch on Nov 10, 2010

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WordPress Plugin Development For Beginners — Presentation Transcript