Extend WordPress with plugins using this advanced WordPress development book, updated for the current version
This significantly updated edition of Professional WordPress Plugin Development addresses modern plugin development for WordPress, the highly popular content management system (CMS). If you’re using WordPress to create and manage websites, WordPress plugins are the software that can extend or enhance CMS functionality. This book offers guidance on writing plugins for WordPress sites to share or sell to other users.
The second edition of Professional WordPress Plugin Development covers the building of advanced plugin development scenarios. It discusses the plugin framework and coding standards as well as dashboards, settings, menus, and related application programming interfaces (APIs). Additional topics include security, performance, data validation, and SQL statements.
• Learn about the power of hooks in WordPress
• Discover how JavaScript and Ajax will work in your site
• Understand key technologies: Block Editor/Gutenberg, JS/React, PHP, and the REST API
• Create and use custom post types and taxonomies.
• Creating custom dashboard menus and plugin settings
• Work with users and user data
• Schedule tasks and utilizing Cron
• Performance and security considerations
Written by experienced plugin developers, Professional WordPress Plugin Development also helps you internationalize and localize your WordPress website. Find out about debugging systems and optimizing your site for speed. As WordPress use continues to increase, you can elevate your professional knowledge of how to extend WordPress through plugins.
Table of Contents
Introduction xxxi
Chapter 1: An Introduction to Plugins 1
What is a Plugin? 1
How Plugins Interact with WordPress 2
When are Plugins Loaded? 3
Available Plugins 4
Official Plugin Directory 4
Popular Plugin Examples 4
Popular Plugin Tags 5
Advantages of Plugins 5
Not Modifying Core 5
Why Reinvent the Wheel? 5
Separating Plugins and Themes 6
Easy Updates 7
Easier to Share and Reuse 7
Plugin Sandbox 7
Plugin Community 7
Installing and Managing Plugins 8
Installing a Plugin 8
Managing Plugins 9
Editing Plugins 9
Plugin Directories 9
Types of Plugins 10
Summary 11
Chapter 2: Plugin Framework 13
Requirements for Plugins 13
Naming Your Plugin 14
Using a Folder 14
Best Practices 14
Namespace Everything 14
File Organization 15
Folder Structure 16
Plugin Header 17
Creating the Header 17
Plugin License 18
Determining Paths 19
Plugin Paths 19
Local Paths 19
URL Paths 20
Activate/Deactivate Functions 22
Plugin Activation Function 22
Plugin Deactivation Function 23
Deactivate is Not Uninstall 23
Uninstall Methods 24
Why Uninstall is Necessary 24
Uninstall.php 24
Uninstall Hook 25
Coding Standards 25
Document Your Code 26
Naming Variables and Functions 27
Naming Classes and Methods 27
Naming Files 28
Single and Double Quotes 28
Indentation 28
Brace Style 29
Space Usage 29
Shorthand PHP 30
SQL Statements 30
Summary 30
Chapter 3: Dashboard And Settings 31
Adding Menus and Submenus 31
Creating a Top-Level Menu 32
Adding a Submenu 33
Adding a Menu Item to an Existing Menu 34
Plugin Settings 36
The Options API 36
Saving Options 36
Saving an Array of Options 37
Updating Options 37
Retrieving Options 38
Loading an Array of Options 39
Deleting Options 40
The Autoload Parameter 40
Segregating Plugin Options 41
Toggling the Autoload Parameter 42
The Settings API 42
Benefits of the Settings API 42
Settings API Functions 43
Creating the Plugin Administration Page 43
Registering New Settings 44
Defining Sections and Settings 44
Validating User Input 46
Rendering the Form 47
All Done! 48
Wrapping It Up: A Complete Plugin Management Page 48
Improving Feedback on Validation Errors 51
Expanding with Additional Field Types 52
Adding Fields to an Existing Page 59
How It Works 59
Adding a Section to an Existing Page 59
Adding Only Fields 60
WordPress’ Sections and Setting Fields 61
User Interface Concerns 62
Removing Settings 62
Keeping It Consistent 63
Using the WordPress UI 64
Headings 64
Dashicons 64
Messages 65
Buttons 66
Form Fields 67
Tables 68
Pagination 69
Summary 70
Chapter 4: Security And Performance 71
Security Overview 72
What Securing Your Plugin is 72
What Securing Your Plugin is Not 72
User Permissions 72
How to Check current_user_can() 73
Do Not Check Too Early 73
Nonces 74
Authority vs. Intention 74
What is a Nonce? 75
How to Create and Verify Nonces 75
Creating a Nonce URL 76
Creating a Nonce Field 76
Creating and Verifying a Nonce in a Plugin 77
Data Validation and Sanitization 78
The Need for Data Validation and Sanitization 78
Good Practice: Identifying Potentially Tainted Data 80
Validating or Sanitizing Input? 81
Validating and Sanitizing Cookbook 81
Integers 81
Arbitrary Text Strings 82
Key and Identifier Strings 83
Email Strings 84
URLs 84
HTML 86
JavaScript 88
Environment and Server Variables 88
Arrays of Data 89
Database Queries 89
Formatting SQL Statements 90
The $wpdb Object 90
Why wpdb Methods are Superior 91
All-in-One Methods 91
$wpdb->update() 92
$wpdb->insert() 92
Common Methods 93
Select a Variable 93
Select a Row 94
Select a Column 94
Select Generic Results 95
Generic Queries 96
Protecting Queries against SQL Injections 96
Security Good Habits 97
Performance Overview 97
Caching 98
Saving Cached Data 98
Loading and Using Cached Data 99
Deleting Cached Data 99
Caching Data within a Plugin 100
Transients 101
Saving an Expiring Option 102
Retrieving an Expiring Option 102
Deleting an Expiring Option 102
A Practical Example Using Transients 102
Technical Details 103
Transient Ideas 103
Summary 104
Chapter 5: Hooks 105
Understanding Hooks 105
Actions 106
What is an Action? 107
Action Hook Functions 108
remove_action() 108
remove_all_actions() 109
do_action_ref_array 110
has_action 111
did_action() 112
current_action 112
register_activation_hook and register_deactivation_hook 113
Commonly Used Action Hooks 113
plugins_loaded 113
init 114
admin_menu 115
save_post 116
wp_head 116
Filters 117
What is a Filter? 118
Filter Hook Functions 119
remove_filter 119
remove_all_filters 120
apply_filters_ref_array 121
has_filter 122
current_filter 122
Quick Return Functions 123
Commonly Used Filter Hooks 124
the_content 124
template_include 125
Using Hooks from within a Class 126
Using Hooks with Anonymous Functions 127
Creating Custom Hooks 128
Benefits of Creating Custom Hooks 128
Custom Action Hook Example 128
Custom Filter Hook Example 129
Finding Hooks 129
Searching for Hooks in the Core Code 130
Variable Hooks 130
Hook Reference Lists 130
Summary 131
Chapter 6: Javascript 133
Registering Scripts 134
Enqueueing Scripts 135
Limiting Scope 136
Localizing Scripts 136
Inline Scripts 137
Overview of Bundled Scripts 139
jQuery UI and Other Scripts 139
The WP Global 140
a11y Speak 140
Escaping 140
i18n 140
Heartbeat 141
Polyfills 141
Your Custom Scripts 141
jQuery 142
Benefits of Using jQuery 142
jQuery Crash Course 142
The jQuery Object 143
Syntax and Chaining 143
No-Conflict Mode in WordPress 144
Launching Code on Document Ready 144
Ajax 145
Backbone/Underscore 146
React 146
Summary 147
Chapter 7: Blocks And Gutenberg 149
What is Gutenberg? 149
Touring Gutenberg 151
Practical Examples 155
WooCommerce 156
The Events Calendar 157
Post Type Switcher 158
Technology Stack of Gutenberg 159
JavaScript 160
PHP 161
Node.js 162
webpack 163
Babel 163
React 163
JSX 163
ES6 163
“Hello World!” Block 164
PHP 164
JavaScript 164
webpack 165
Command Line 166
Activation 167
Wrap-Up 167
WP-CLI Scaffolding 168
Plugin 168
Blocks 169
Includes 169
Activation 169
Wrap-Up 169
create-guten-block Toolkit 170
Installation 171
Activation 171
Wrap-Up 173
Block Directory 173
Summary 174
Chapter 8: Content 175
Creating Custom Post Types 176
Post Type Possibilities 176
Registering a Post Type 177
register_post_type 177
Registering the Book Collection Post Type 177
Setting Post Type Labels 179
Using Custom Capabilities 180
Attaching Existing Taxonomies 182
Post Metadata 183
Registering Post Metadata 183
Adding Post Metadata 184
Retrieving Post Metadata 185
Updating Post Metadata 186
Deleting Post Metadata 186
Meta Boxes 187
What is a Meta Box? 187
Adding a Custom Meta Box 188
Saving Meta Box Data 190
Creating Custom Taxonomies 191
Understanding Taxonomies 192
Registering a Custom Taxonomy 192
register_taxonomy 193
Registering the Genre Taxonomy 193
Assigning a Taxonomy to a Post Type 194
Using Custom Taxonomies 195
Retrieving a Taxonomy 196
Using a Taxonomy with Posts 196
Taxonomy Conditional Tags 197
taxonomy_exists 197
is_taxonomy_hierarchical 198
is_tax 198
A Post Type, Post Metadata, and Taxonomy Plugin 199
Summary 204
Chapter 9: Users And User Data 205
Working with Users 206
User Functions 206
is_user_logged_in() 206
get_users() 207
count_users 207
Creating, Updating, and Deleting Users 208
Creating a New User 209
Updating an Existing User 211
Deleting an Existing User 212
User Data 213
Getting a User Object and Data 213
Getting the Current User Object 215
Getting User Post Counts 215
User Metadata 217
Adding User Metadata 218
Retrieving User Metadata 218
Updating User Metadata 219
Deleting User Metadata 220
Creating a Plugin with User Metadata 220
Roles and Capabilities 223
What are Roles and Capabilities? 223
Default Roles 224
Custom Roles 224
Limiting Access 225
Checking User Permissions 225
Is the User an Admin? 227
Customizing Roles 228
Creating a Role 228
Deleting a Role 230
Adding Capabilities to a Role 230
Removing Capabilities from a Role 231
A Custom Role and Capability Plugin 231
Summary 233
Chapter 10: Scheduled Tasks 235
What is Cron? 235
How is Cron Executed? 236
Scheduling Cron Events 236
Scheduling a Recurring Event 236
Scheduling a Single Event 238
Unscheduling an Event 240
Specifying Your Own Cron Intervals 241
Viewing Scheduled Cron Events 242
True Cron 247
Practical Use 247
The Blog Pester Plugin 247
Deleting Post Revisions Weekly 250
The Delete Comments Plugin 253
Summary 258
Chapter 11: Internationalization 259
Internationalization and Localization 260
Why Internationalize? 260
Understanding Internationalization in Professional Work 261
Getting Your Plugin Ready for Translation 261
Echoing and Returning Strings 262
The __() Function 262
The _e() Function 263
The esc_attr__() Function 263
The esc_attr_e() Function 264
The esc_html__() Function 264
The esc_html_e() Function 264
The _x() Function 264
The _ex() Function 265
The esc_attr_x() Function 266
The esc_html_x() Function 266
The _n() Function 267
The _nx() Function 268
The _n_noop() Function 269
The _nx_noop() Function 270
Using Placeholders 271
Internationalizing JavaScript 273
Developer Handbook Resource 275
Creating Translation Files 275
The MO and PO Files 276
Translation Tools 276
How to Create a POT File 277
Command Line 278
Where to Store Translation Files 278
Summary 278
Chapter 12: Rest Api 279
What the REST API is 279
What You Can Do with the REST API 280
Accessing the WordPress REST API 281
Default Endpoints 282
REST API Clients 283
Insomnia 283
Postman 283
Authentication 284
Enhanced Authentication 285
Custom Endpoints 286
The HTTP API 289
What is an HTTP Request? 289
HTTP Request Concepts 289
Dissecting an HTTP Transaction 289
Some Caveats on Checking HTTP Responses 291
Possibilities for Crafting HTTP Requests 292
How to Make HTTP Requests in PHP 292
Using the HTTP Extension 292
Using fopen() Streams 293
Using a Standard fopen() 293
Using fsockopen() 293
Using the CURL Extension 294
Too Many Ways? 294
WordPress’ HTTP Functions 294
The wp_remote_ Functions 294
wp_remote_* Input Parameters 295
wp_remote_* Return Values 297
wp_remote_ Companion Functions 302
Advanced Configuration and Tips 303
Proxy Support 303
Filtering Requests and Responses 304
Bringing It All Together 307
Create 307
Update 309
Delete 309
Resources 313
Summary 313
Chapter 13: Multisite 315
Terminology 316
Advantages of Multisite 317
Enabling Multisite in WordPress 317
Multisite Functions 319
The Site ID 319
Common Functions 319
Switching and Restoring Sites 321
Network Content Shortcode Examples 325
A Network Content Widget Example 330
Creating a New Site 336
Site Options 342
Network Options 342
Site Meta 343
Users and Roles 343
Super Admin 348
Checking the Site Owner 349
Network Stats 350
Database Schema 350
Multisite-Specific Tables 350
Site-Specific Tables 351
Query Classes 351
WP_Site_Query 351
WP_Network_Query 352
Object Classes 352
WP_Site 352
WP_Network 352
Summary 352
Chapter 14: The Kitchen Sink 353
Querying and Displaying Posts 353
Use Case for Displaying Posts 354
WP_Query Overview 354
The Loop 355
Shortcodes 358
What Shortcodes are 358
Register Custom Shortcodes 359
Building a Simple Shortcode 360
Building a Shortcode with Parameters 362
Building a Shortcode with Content 364
Shortcode Tips 366
Think Simplicity for the User 366
Remember That Shortcodes are Dynamic 366
Look under the Hood 367
remove_shortcode() 367
remove_all_shortcodes() 367
strip_shortcodes() 367
do_shortcode() 367
Widgets 368
Creating a Widget 368
Dashboard Widgets 377
Creating Dashboard Widgets 378
Creating a Dashboard Widget with Options 380
Rewrite Rules 384
Why Rewrite URLs 384
Permalink Principles 384
Search Engine Friendly 384
User Friendly 385
Apache’s mod_rewrite 385
URL Rewriting in WordPress 385
How WordPress Handles Queries 386
Overview of the Query Process 386
The Rewrite Object 387
The Query Object 388
What Plugins Can Do 389
Practical Uses 389
Rewriting a URL to Create a List of Shops 389
Creating a New Permalink Structure and Integrating Non-WordPress Pages 391
The Heartbeat API 395
What is the Heartbeat API? 395
Using the Heartbeat API 395
Sending Data 395
Receiving and Responding to Data 395
Processing the Response 396
Full Heartbeat API Plugin 396
Summary 398
Chapter 15: Debugging 399
Compatibility 399
Supporting Many WordPress Versions 400
Playing Nicely with Other WordPress Plugins 401
Keeping Current with WordPress Development 402
Deprecation 404
Dealing with Obsolete Client Installs 404
Debugging 405
Enabling Debugging 406
Displaying Debug Output 406
Understanding Debug Output 407
Error Logging 410
Enabling Error Logging 410
Setting Log File Location 411
Understanding the Log File 411
Query Monitor 411
Summary 414
Chapter 16: The Developer Toolbox 415
Core as Reference 415
PHP Inline Documentation 416
JavaScript Inline Documentation 417
Finding Functions 417
Common Core Files 418
formatting.php 418
functions.php 418
pluggable.php 419
plugin.php 419
post.php 419
user.php 420
Plugin Developer Handbook 420
Navigating the Handbook 420
Code Reference 420
Codex 421
Searching the Codex 421
Function Reference 422
Tool Websites 422
PHPXref 422
Hooks Database 423
Community Resources 424
Make WordPress 424
Support Forums 424
WordPress Slack 425
WordPress Development Updates 425
WordPress Ideas 426
Community News Sites 426
WordPress News 426
WordPress Planet 426
Post Status 426
Know the Code 426
LinkedIn Learning 427
Twitter 427
Local Events 427
Tools 427
Browser 427
Editor 428
NetBeans IDE 428
PhpStorm 428
Notepad++ 428
TextMate 428
Sublime Text 428
Visual Studio Code 429
Deploying Files with FTP, SFTP, and SSH 429
phpMyAdmin 429
Summary 430
Index 431
