WordPress (dev)

# show error on screen
define('WP_DEBUG', true)

# disable revision
define('WP_POST_REVISIONS', false)

# for debugging theme
define('SAVEQURIES', true)
# in theme
if (current_user_can('manage_options')) {
    global $wpdb;
    print_r($wpdb->quries);
}

# force SSL
define('FORCE_SSL_LOGIN', true)
define('FORCE_SSL_ADMIN', true)

Security

Security Measures to keep WordPress powered Websites more Secure | WordPress Arena

22 Must Follow WordPress Security Tips - Bloggersignal

Important Security Fix for WordPress | Perishable Press

Changing File Permissions « WordPress Codex

How To Change Wordpress Default 'Admin" Username

How to prevent access to WordPress Administration Menu items | WordPress Arena

Making your WordPress site More Secure by Adding HTTPS and SSL | WordPress Arena
How To Use SSL & HTTPS With WordPress | Elegant Themes Blog
Options for SSL in WordPress - Tuts+ Code Tutorial
SSL and Cookies in WordPress 2.6 | Ryan Boren

Secure WordPress with IP restricted access
How to Restrict IP Addresses to Login on WordPress Dashboard
How to Limit Access by IP to Your wp-login.php file in WordPress

In .htaccess file of the WordPress directory:

# Limit access by IP address
<Files wp-login.php>
        order deny,allow
        Deny from all

# whitelist IP address one
allow from xx.xxx.xx.xxx

# whitelist IP Address two
allow from xx.xxx.xx.xxx
</Files>

Insert Sample Data

Function Reference « WordPress Codex
Function Reference/wp insert category « WordPress Codex
Function Reference/wp insert term « WordPress Codex
Function Reference/wp insert post « WordPress Codex
Function Reference/wp trash post « WordPress Codex

themes/exclusive/admin/install_sampl_date.php

//Check if category already exists
$cat_ID = get_cat_ID( $category );

//If it doesn't exist create new category
if($cat_ID == 0) {
    $my_cat = array(
        'cat_name' => 'My Category',
        'category_description' => 'A Cool Category',
        'category_nicename' => 'category-slug',
        'category_parent' => '',
        'taxonomy' => 'category'
    );

    $my_cat_id = wp_insert_category($my_cat);
}

// this is practically what wp_insert_category() does underneath
if(!term_exists('sample-category')) {
    wp_insert_term(
        'Sample Category',
        'category',
        array(
          'description' => 'This is an sample category.',
          'slug'        => 'sample-category'
        )
    );
}

Plugins

Plugins « WordPress Codex

WordPress › WordPress Plugins
Writing a Plugin « WordPress Codex
Plugin API « WordPress Codex
How To Improve WordPress Plugins - WP Engineer

The WordPress Plugin Boilerplate | A Foundation For Building High-Quality WordPress Plugins
Developing Plugins With WordPress Boilerplates: Why Boilerplates Matter - Tuts+ Code Article
Developing Plugins With WordPress Boilerplates: Building a Plugin - Tuts+ Code Article

Aesop Story Engine
Helpful Wordpress Plugins | Linux.org
Top 20 Useful WordPress ShortCode Plugins | Web Development Tutorials and Resources @ ScratchingInfo

Themes

Theme Development « WordPress Codex
WordPress › Getting Started | Theme Developer Handbook | WordPress Developer Resources
A Beginners Guide to the WordPress Template Hierarchy | Learn WordPress

Theme Customization API « WordPress Codex
Stepping into Templates « WordPress Codex
Template Tags « WordPress Codex
Theme Features « WordPress Codex

Writing Maintainable WordPress Themes - Tuts+ Code Tutorials
Freemius - The new standard in selling WordPress plugins and themes

Writing Maintainable WordPress Themes | Tom McFarlin
Separation of Concerns with WordPress Templates | Tom McFarlin
A Rule of Thumb for WordPress Partials | Tom McFarlin
Adding Scripts and Styles to WordPress the Right Way With Enqueueing - WPMU DEV

10 Checks to the Perfect WordPress theme - WP Engineer

Widgetizing Themes — Automattic
Widgetizing Themes « WordPress Codex

Easily Adaptable WordPress Loop Templates: Basic Loops, Mullet Loops, and More.. | Perishable Press

Also see Widgets

Links

categories - Direct link to category? - WordPress Development Stack Exchange
Linking Posts Pages and Categories « WordPress Codex

Customizing Theme

Deactivate WordPress Default Widgets - WP Engineer

WordPress › Customizer Framework « WordPress Plugins
WordPress › Custom Sidebars « WordPress Plugins
WordPress › Stag Custom Sidebars « WordPress Plugins
WordPress › Easy Custom Sidebars « WordPress Plugins

Function Reference/register sidebars « WordPress Codex
Function Reference/dynamic sidebar « WordPress Codex

Define sidebar in functions.php:

register_sidebar(array(
    'name' => __('MySidebar', 'theme'),
    'id' => 'my-sidebar',
    'description' => __('this is a custom sidebar', 'theme'),
    'before_widget' => '<div class="sidebar-box">',
    'after_widget' => '</div>',
    'before_title' => '<span class="sidebar-title">',
    'after_title' => '</span><div class="dots"></div>'
))

Add to the desired location in the theme:

<div id="sidebar">
  <?php if ( !dynamic_sidebar("my-sidebar") ) : ?>
  <div class="sidebar-box">
    <span class="sidebar-title">MySidebar</span>
    <p>Add Widgets to the sidebar 'MySidebar'</p>
  </div>
  <?php endif; ?>
</div>
<!-- sidebar -->

Different sidebar depending on page, usually in sidebar.php:

<?php
<div id="sidebar">
if ( is_front_page() ) { if ( !dynamic_sidebar("front-page-sidebar") ) : ?>
<li>front-page-sidebar is empty</li>
<?php endif; } elseif ( is_category('news') || in_category('news') ) { if (
!dynamic_sidebar("news-sidebar") ) : ?>
<li>news-sidebar is empty</li>
<?php endif; } elseif ( is_single() ) { // is_singular() for both Posts/Pages if
( !dynamic_sidebar("post-sidebar") ) : ?>
<li>post-sidebar is empty</li>
<?php endif; } elseif ( is_page() ) { if ( !dynamic_sidebar("page-sidebar") ) :
?>
<li>page-sidebar is empty</li>
<?php endif; } ?>

Function Reference/register nav menus « WordPress Codex
Function Reference/wp nav menu « WordPress Codex

Define nav_menu in functions.php:

register_nav_menus( array(
    'top-navigation' => __('Top Navigation', 'theme')
) );

Add to the desired location in the theme:

<div id="header-bottom"><?php wp_nav_menu('top-navigation') ?></div>

Customizer

» The WordPress Theme Customizer: a Comprehensive Developer’s Guide
A Guide to the WordPress Theme Customizer - Tuts+ Code Tutorials
Digging Into the Theme Customizer - Tuts+ Code Tutorials

paulund/wordpress-theme-customizer-custom-controls

Customizer: Theme as documentation
bueltge/Documentation
The Customizer on Vimeo
WordPress Theme Customizer Custom Controls - WP Engineer

Framework

Thematic, A WordPress Theme Framework | ThemeShaper
Hybrid Core WordPress theme framework
Whiteboard Framework for WordPress
Roots | Modern WordPress Development
Wonderflux - WordPress free, open source theme framework
Astra – Fast, Lightweight & Customizable Free WordPress Theme - Download Now!

How Theme Frameworks Actually Work - Tuts+ Code Tutorials

Theme Hybrid: A WordPress theme club
Bones - The HTML5 Wordpress Starter Theme
Responsive WordPress Theme WP-Forge

ThemeShaper

The ThemeShaper WordPress Theme Tutorial: 2nd Edition | ThemeShaper
What Do You Really Need in a WordPress Starter Theme? | ThemeShaper
Underscores.me — The Best Way To Get Started With The _s Theme | ThemeShaper
Thematic, A WordPress Theme Framework | ThemeShaper
Automattic/_s

The Loop

The Loop selects content from MySQL database (typically with query based on URL). It is the link between the theme and the contents in database. WordPress is using the Loop anywhere content is displayed.

Simplest Loop:

// these functions uses global variable `WP_Query $wp_query`
if ( have_posts() ) :
    while ( have_posts() ) :
        the_post();
        // content is available
    endwhile;
endif;

Writing Clean, Maintainable Custom WordPress Queries | Tom McFarlin
How To Setup Custom Queries For WP_Query Pagination | Tom McFarlin
Separation of Concerns: Queries and Helper Functions | Tom McFarlin

wp query - When should you use WP_Query vs query_posts() vs get_posts()? - WordPress Development Stack Exchange

Mastering WP_Query - Tuts+ Code Tutorials

Settings

Settings API « WordPress Codex
Sanitization with the WordPress Settings API | Tom McFarlin
The Complete Guide To The WordPress Settings API - Tuts+ Code Tutorials

Child Themes

You can easily inherit from an existing theme and override the functions.
But the theme may not be child friendly (e.g.: using get_template_directory() instead of get_stylesheet_directory(), not using get_template_part()). And my experience is that overriding files may cause the child theme to breakdown altogether.
I now prefer make a copy of the desired theme, change the code directly and track the modification via git.

Child Themes « WordPress Codex
How To Modify WordPress Themes The Smart Way | ThemeShaper
How to Create a WordPress Child Theme - WPMU DEV

What is a WordPress Child Theme? Pros, Cons, and More
WordPress › Support » Overriding parent theme sub-files with a child theme. HOW?
WordPress › Support » Child Themes and PHP pages

Widgets

WordPress Widgets « WordPress Codex

Build A WordPress 2.8 Widget With The New Widget API - WP Engineer

Embedding React

Taxonomies

The Beginner’s Guide to WordPress Taxonomies - Tuts+ Code Tutorials
The Tuts+ Guide to Template Tags - Tuts+ Code Tutorials
A Walkthrough on Conditional Tags in WordPress - Tuts+ Code Tutorials

Custom Post Type UI – WordPress plugin | WordPress.org

Tuts+

WordPress - Tuts+ Code Category
Tools of the Smart WordPress Developer - Tuts+ Code Tutorials

The Complete Guide To The WordPress Settings API - Tuts+ Code Tutorials
Making the Perfect WordPress Theme - Tuts+ Code Tutorials
Quick Tips to Boost Your WordPress Website's Speed - Tuts+ Code Tutorials
A Beginner’s Guide to Using WordPress - Tuts+ Course
The Beginner's Guide to Selecting a WordPress Theme - Tuts+ Code Article
Trim the Bloat: Keeping WordPress Healthy - Tuts+ Code Tutorials
Fifty Actions of WordPress - Tuts+ Code Tutorials
Understanding and Working with Data in WordPress - Tuts+ Code Tutorials

WordPress (dev)

Resources

WP-CLI

WP API (PHP)

WP API (REST)

Calypso (admin on REST API)

WordPress Blocks

WPGraphQL

Headless WordPress themes

Fresh WordPress Themes

Frontity

Headless CMS

WordPress Dev Environment

Jetpack

Multisite

Sites

Books

Tips

`wp-config.php`

Security

Insert Sample Data

Plugins

Themes

Links

Customizing Theme

Adding Sidebar

Adding Nav Menu

Customizer

Framework

ThemeShaper

The Loop

Settings

Child Themes

Widgets

Embedding React

Taxonomies

Tuts+

WordPress (dev)

Resources

WP-CLI

WP API (PHP)

WP API (REST)

Calypso (admin on REST API)

WordPress Blocks

WPGraphQL

Headless WordPress themes

Fresh WordPress Themes

Frontity

Headless CMS

WordPress Dev Environment

Jetpack

Multisite

Sites

Books

Tips

wp-config.php

Security

Restricted Login Access

Insert Sample Data

Plugins

Themes

Links

Customizing Theme

Adding Sidebar

Adding Nav Menu

Customizer

Framework

ThemeShaper

The Loop

Settings

Child Themes

Widgets

Embedding React

Taxonomies

Tuts+

`wp-config.php`