This document provides an introduction to plugin development in WordPress from Josh Pollock of CalderaLabs. It discusses what plugins are, why developers should create them, how to structure them, and best practices for the plugin API. Key topics covered include using hooks and filters, plugin organization, and prioritizing security, reuse, and good coding standards.

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Introduction To Plugin
Development
Josh Pollock | CalderaLabs.org

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CalderaLabs.org
Hi I'm Josh
Lead Developer: CalderaWP
I make WordPress plugins
I teach about WordPress
I wrote a book about the WordPress REST API
I am core contributor to WordPress
I am a member of The WPCrowd

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CalderaLabs.org
You Should Make Plugins

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CalderaLabs.org
Turn Code Off
Easy troubleshooting!

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CalderaLabs.org
Reuse Code
Share Between Projects

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CalderaLabs.org
Share Code
Distribute on Github or WordPress.org

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CalderaLabs.org
Make $$
Sell Plugins

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CalderaLabs.org
What Is A Plugin?

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How To Make A Plugin
Step 1: Add a new directory to the plugins directory
(optional).
Step 2: Add a file in that directory or the plugins
directory with a valid plugin header.
Step 3: Write code (optional).

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CalderaLabs.org
Plugins Are
Containers For Code

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CalderaLabs.org
Plugins vs Themes
Or Can’t I Put That In
functions.php ??

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CalderaLabs.org
It’s a free software, you
can do whatever you
want, but...

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Plugins vs Themes: Best Practices
Themes should be use to present a unique design
Everything else should be in a plugin.

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Should This Code Go In functions.php ?
Is the answer to any of these questions no:
Do I ever want to reuse this code apart from this
theme?
Will I miss this code when I switch themes?
Switch for troubleshooting.
Switch for new look.
Can I use this code for other purposes?

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CalderaLabs.org
Making Plugins
Organization

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Plugin Header: Goes In Main File
<?php
/*
Plugin Name: My Plugin
*/

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Main File: The File With A Plugin Header
Plugin Header
License/ Copyright/ Contact Info
Check Dependencies & Requirements
Load Plugin
Start Plugin

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Directory Structure
There Are No Rules
You Should Have Rules
I like PSR-4 :)

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Naming Things
There Are No Rules
You Should Have Rules
I like to document naming
conventions:)

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Naming Things: naming-conventions.txt
Plugin Name --
Text domain --
Function prefix --
Class prefix --
Root namespace --
Hook Prefix --

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CalderaLabs.org
Making Plugins
The Plugins API

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CalderaLabs.org
Hooks Are Events
When We Get Here:
Do This

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CalderaLabs.org
Don’t
Hack Core
Modifying Core/ Plugins / Themes Is A Bad Idea
You Will Lose Your Changes On Update

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CalderaLabs.org
Hooks Let You Change
WordPress/ Plugins/
Themes
Without Editing Files

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CalderaLabs.org
Filters
Change A Variable’s Value
At A Specific Time

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CalderaLabs.org
Actions
Do Something
At A Specific Time

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Actions
Do something
Don’t return values*
Actions Vs Filters
Filters
Change something
Must return a value*

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CalderaLabs.org
Use Hooks In Your Plugins
To Add New Functionality

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CalderaLabs.org
Add Hooks In Your Plugins
To Make Your Plugin
Extensible

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do_action()
Triggers the action
Using Actions
add_action()
Hooks a function to an action

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Using Actions: Print Tracking Pixel
function my_prefix_pixel() {
echo '<img src="https://adnetwork.com/pixel.gif"
width="0" height="0">';
}
add_action( 'wp_footer', 'my_prefix_pixel' );

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Using Actions: Modify WP_Query
function search_filter($query) {
if ( !is_admin() && $query->is_main_query() ) {
if ($query->is_search) {
$query->set('post_type', array( 'post', 'movie' ) );
}
}
}
add_action('pre_get_posts','search_filter');

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Using Actions

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apply_filters()
Creates the filter
Using Filters
add_filter()
Hooks a function to a filter

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Using Filters: Add To Content
add_filter( 'the_content', 'slug_callback' );
function slug_callback( $content ){
return $content . '<p class="cta">Hey Sign Up for My
Thing!</p>';
}

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Using Filters: Add To Content
add_filter( 'the_content', function( $content ){
return $content . '<p class="cta">Hey Sign Up for
My Thing!</p>';
});

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Creating Filters: Before
function prefix_get_form( $id ){
return get_option( $id, [] );
}

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Creating Filters: After
function prefix_get_form( $id ){
$form = get_option( $id, [] );
$form = apply_filters( 'prefix_get_form', $form, $id );
return $form;
}

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Hook Priority
add_filter( 'name','callback', 2 );
add_filter( 'name', 'callback_two', 55 );

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Hook Arguments
$form = apply_filters( 'prefix_get_form', $form, $id );
add_filter( 'prefix_get_form', function( $form, $id ){
if( 'CF1234567' == $id ){
// do something to $form
}
return $form;
}, 10, 2 );

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CalderaLabs.org
Making Plugins
Josh’s Rules For Plugin
Development

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CalderaLabs.org
Security Is Not
Optional

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CalderaLabs.org
Trust No Inputs

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HTTP requests
That functions will be used how you intended
them
Data returned from the database
Yourself
Some Things Not To Trust

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CalderaLabs.org
Write For Reuse

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CalderaLabs.org
Use Version
Control

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CalderaLabs.org
Commit Early
Commit Often

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Single Responsibility Principle
Functions should do one thing.
Do Not Repeat Yourself (DRY)
Functions, not copypaste
Making Code Reusable

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CalderaLabs.org
Abstraction!!

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CalderaLabs.org
Using Classes
Doesn’t Make Your
Code
Object-Oriented

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CalderaLabs.org
Use
Object-Oriented
Programming
When Appropriate

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CalderaLabs.org
Small Simple
Classes Are Good

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CalderaLabs.org
Josh Pollock
JoshPress.net
CalderaLabs.org
CalderaWP.com
IngotHQ.com
@Josh412