Write your first WordPress plugin

Write your first
WordPress plugin
Anthony Montalbano @italianst4

anthony@ambrdetroit.com

Who is Anthony Montalbano?
Passionate for code
Bachelor's in Computer Science
Passionate for WordPress
WordCamp Detroit Organizer

Passionate for open source
WordPress plugin developer

Passionate for words
Serial blogger

Passionate for possibilities
Co-founder of flipfrog and AMBR Detroit

What is a WordPress plugin?
"Plugins are tools to extend the functionality of WordPress."
~ http://codex.wordpress.org/Plugins

MEGA IMPORTANT!
The Codex

The online manual for WordPress and a living
repository for WordPress information and
documentation.

WordPress Plugins by the Numbers

21,214 345,389,937
# of plugins # of plugin downloads

63
% of users that use
10 plugins or less 9,783+
# of plugin
12,134,168 developers
# of downloads of the most popular
plugin - All in One SEO Pack
Sources:
http://wordpress.org/extend/plugins/
http://digwp.com/2010/01/poll-results-how-many-plugins-do-you-use/
http://www.daveligthart.com/top-1000-wordpress-plugin-authors/

Your WordPress plugin "Google"

Let's make a plugin!

● Find and replace a word in the title.
● Show what is being replaced as a sidebar widget.
● Admin menu to change the find word and replace word.
● Email me when a new post is published.

Filter Hooks and Action Hooks
WordPress plugins rely on the many hooks
within the system to get things done.
Filter Hooks
Filters are the hooks that WordPress launches to modify
text of various types before adding it to the database or
sending it to the browser screen.

Action Hooks
Actions are the hooks that the WordPress core launches at
specific points during execution, or when specific events
occur.
Source: http://codex.wordpress.org/Plugin_API

How Hooks Work
WordPress Plugin API provides you with a set
of PHP functions that allow you to signal your
own functions to be called when that hook is
called.

Filter Example:
add_filter('the_title', function($title) { return '<b>'.
$title. '</b>';})
Filters: http://codex.wordpress.org/Plugin_API/Filter_Reference

Action Example:
add_action( 'save_post', 'my_save_post', 10, 2 );
Actions: http://codex.wordpress.org/Plugin_API/Action_Reference

How Hooks Work (continued)
Hooks have 4 parameters
● Tag (required)
○ This is the WordPress named location where the
hook takes place.
● Function (required)
○ This is the function to be called when the hook is
executed.
● Priority (optional)
○ This determines the order your function is run, the
lower, the earlier.
● Parameters (optional)
○ This is the number of parameters your function takes

Set the Foundation
● Create a new folder in wp-content/plugins
● Create a php file with a plugin header
comment box
/*
Plugin Name: My First Plugin
Plugin URI: http://wordpress.org/extend/plugins/
Description: This is a description of a plugin
Author: Anthony Montalbano
Version: alpha
Author URI: http://www.ambrdetroit.com
*/

http://codex.wordpress.org/Writing_a_Plugin#File_Headers

Activation and Uninstall
What will your plugin do when it is first
activated?
● Create database tables, data, and files
● Update database tables, data, and files

What will your plugin do when it is uninstalled?
● Delete databases tables, data, files

On Activation
Add the following hook:
register_activation_hook( __FILE__, 'demo_activate' );

Create a new function called 'demo_activate'
function demo_activate() {
//do something when the plugin first initializes

};

Source: http://codex.wordpress.org/Function_Reference/register_activation_hook

On Uninstall
Create a file called uninstall.php in the root directory.

Add the following code:
<?php
if(!defined('WP_UNINSTALL_PLUGIN'))
exit();

delete_option('demo_myValue');

Source: http://codex.wordpress.org/Function_Reference/register_deactivation_hook

Adding a Filter
Add the following filter hook:
add_filter( 'the_title, 'demo_title_change' );

Create a new function called 'demo_title_change'
function demo_title_change($title) {
//do something with the title
str_replace( 'world', 'something', $title);
return $title;
};

Source: http://codex.wordpress.org/Function_Reference/add_filter

Adding an Action
Add the following filter hook:
add_action( 'publish_post, 'demo_email_me' );

Create a new function called 'demo_title_change'
function demo_email_me($post_id) {
wp_mail('anthony@ambrdetroit.com', 'New post!', 'New
post on my demo blog, go check it out:' . get_bloginfo
('url'));
return $post_id;
};

Source: http://codex.wordpress.org/Function_Reference/add_filter

Adding a Settings Page
First we need add a hook to where the settings
page will show in the admin:
add_action('admin_menu', 'my_plugin_menu');

Next we need to add a function to define the
menu:
function my_plugin_menu() {
add_options_page('Demo Plugin Options', 'Demo
Plugin', 'manage_options', 'demo-plugin',
'demo_plugin_options');
}

Source: http://codex.wordpress.org/Function_Reference/add_options_page

Adding a Settings Page (continued)
Finally we need to generate the HTML and
functionality of the admin menu:
function demo_plugin_options() {
//get the option
$replaceWord = get_option('demo_myValue');

//save functionality
if(isset($_REQUEST['demo_update_admin']) && $_REQUEST
['demo_update_admin']) {
update_option('demo_myValue', $_POST['myValue']);
$replaceWord = $_POST['myValue'];
echo "<div id='message' class='updated fade'><p>Demo Plugin
Settings Saved!</p></div>";
}

//display the page
include_once(dirname(__FILE__) . '/demo_admin.php');
}

Adding a Widget
First we need to add a hook to load the widget
on widget initialization:
add_action( 'widgets_init', 'demo_load_widgets' );

Next, we need to create a function to register
the widget:
function demo_load_widgets() {
register_widget( "demo_widget" );
}

Source: http://codex.wordpress.org/Widgets_API

Adding a Widget (continued)
Finally we create the widget by extending the
WordPress Widget class:
class Demo_Widget extends WP_Widget {

public function __construct() {
// widget actual processes
parent::__construct(
'demo_widget', // Base ID
'Demo Widget', // Name
array( 'description' => __( 'My Little Demo Widget', 'text_domain' ), ) //
Arrgy
);
}

public function widget( $args, $instance ) {
// outputs the content of the widget
extract( $args );
$replaceWord = get_option('demo_myValue');

echo $before_widget;
if ( ! empty( $replaceWord ) )
echo $before_title . 'My value' . $after_title . $replaceWord;
echo $after_widget;
}
}

Tip 1: Use a plugin prefix
When creating a plugin, create a unique plugin
prefix that can be used for all functions and
variables.

Since there are many plugins, it's important that
your functions and variables don't conflict with
other plugins.

Tip 2: Never use PHP MySQL calls
WordPress has a great database class called
WPDB and makes it very each to plugin to the
WordPress database.
http://codex.wordpress.org/Class_Reference/wpdb

For simple name/value pairs you can use
WordPress options
http://codex.wordpress.org/Function_Reference/add_option

Tip 3: Queuing Scripts and Styles
There are many cases where you may want to
include a javascript or style sheet with your
plugin. WordPress has this functionality built
in. By default WordPress has many scripts
included, such as jQuery.

http://codex.wordpress.org/Function_Reference/wp_enqueue_script

Tip 4: Use WordPress Admin Styles
The WordPress admin has a style sheet that
should be used when creating admin menus.
The goal is to make your plugin fit seamless
with WordPress.

http://codex.wordpress.org/User:TECannon/UI_Pattern_and_Style_Guide

Tip 5: Prepare your SQL Statements
WordPress Database class has a function
called prepare(). Use this function to properly
prepare your SQL statements.

http://codex.wordpress.
org/Class_Reference/wpdb#Protect_Queries_Against_SQL_Injection_Attacks

There's a Plugin for That

WordCamp Detroit 2010

http://bit.ly/wcdetplugins

Thank you!

Anthony Montalbano

@italianst4

anthony@ambrdetroit.com

Write your first WordPress plugin

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