WordPress Plugin Development for Beginners

John P. BlochSenior Web DeveloperAvendi 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:

Actions:

Both functions take two optional arguments:Priority

Accepted Arguments

How To Fire Hooks

To fire an action:

This fires all functions hooked into that action

To fire a filter:

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 functionDefaults to 1 for both actions and filters

The following would output:

Don't mention Joomla
Don't mention

Some Common Hooks

Actionsinit

template_redirect

admin_init

after_setup_theme

wp_loaded

wp_head

wp_footer

admin_menu

Filtersthe_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:

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 environmentLAMPP Stack (XAMPP http://apachefriends.org)

SubversionWindows: TortoiseSVN or Cygwin

Everything else: CLI

Text Editor

FTP clientMac: Transmit

SSH ClientWindows: Putty

ReferencePHPXref (http://www.phpxref.com)

WordPress Developer Toolbox

Official Support ChannelsWordPress 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 ChannelsWordPress 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 questionsName 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!

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