Useful resources

• Make Sense of WP Query Functions on StackExchange
• Action Reference on WP codex
• Filter Reference on WP codex
• Capabilities on WP codex (current_user_can)
• 10 things every WP developer should know on SmashingMagazine
• WordPressify - A build system designed to automate your WordPress development workflow

Developing themes

It can be a lot of work to start from nothing, therefore it can be useful to start from a boilerplate:

• Customizing the Roots Theme Pure CSS and Sass/Compass
• Cutlass for OO templates with Blade and Gulp
• Roots Modern WordPress Development, using Composer, Boilerplate, Bootstrap and Grunt
• Timber for OO templates with Twig
• Underscores Start Theme

Developing plugins

• The WordPress Plugin Boilerplate

Useful plugins

Development

• Adding WordPress Admin Tables with WP_List_Table on wpmudev
• Advanced Custom Fields for full control of your edit screens & custom field data
  • Extending ACF on Smashing Magazine
• Asynchronous Tasks by TechCrunch
• Meta Box creates an easy API for custom meta fields
• Posts 2 Posts makes post relationships easy
• Redux Framework
• Regenerate thumbnails in batches
• Tonic Starter Theme aims to modernize and organize some aspects of theme development
• WordPress Importer let's you import exported content

Editing

• Advanced Responsive Video Embedder
• Conductor

Security and Logging

• Blackbox
• Security Audit Log
• Wordfence

Content plugins

• ACF powerful fields for developers
• Akismet check's for spam messages
• Gigpress for artists and tour calendars
• WP User Avatar easy custom avatars
• Yoast SEO for better search engine optimalization

Widget plugins

• Jetpack Widget Visibility manage visibility per widget
• Multisite Posts receives posts across sites
• TinyMCE Widget

Admin UI

• Post types reorder using a dragg and drop interface
• White label CMS for a branded experience
• Titan Framework for creating WP options, intro on tutsplus.

Performance

• WordPress Optimization/WordPress Performance
• WordPress Optimization/Caching
• WordPress Performance Improvements That Can Go Wrong
• Theme Performance
• Transients API
• Asynchronous Javascript
• WP Performance Pack
• WP Super Cache
• WP Fastest Cache
• W3 Total Cache
• Wordfence
• 5 of the Most Popular WordPress Caching Plugins on Sitepoint

MultiSite/Network related

• Broadcast MU
• Proper Network Activiation
• ThreeWP Broadcast

Manage remote WordPress installations with ManageWP Worker.

Forms

• Contact Form 7
• Contact Form DB
• Gravity Forms

Multilingual websites

• Codestyling Localization
• Multilingual Press
• Polylang
• Zanto

Snippets

Get a post from another multisite

You can do this:

switch_to_blog( $site_id );
$post = get_post( $post_id );
restore_current_blog();

But this function is also in WP core:

function get_blog_post( $blog_id, $post_id ) {
    switch_to_blog( $blog_id );
    $post = get_post( $post_id );
    restore_current_blog();

    return $post;
}

More background information:

• How to get a post from another site in the network
• Switch to blog performance
• Before you create a network
• Share some data between two blogs

Misc.

• Customize Login Page
• WordPress for Web Apps provides the essential components you need to power your web app with WordPress

Posts

• Get posts
• WP_Query
• Using WP_Query on Smashing Magazine

API

• Theming with the REST API – Meet Picard
• WP API on GitHub
• WP API Docs

Moving WordPress (changing the domain and/or location)

remove_action('template_redirect', 'redirect_canonical');

wp-login.php can be used to (re-)set the URIs. Find this line:

require( dirname(__FILE__) . '/wp-load.php' );

and insert the following lines below:

// Comment or remove these lines of code once the database is updated,
// due to performance. Good to know: they don't kick in when the url and
// home are defined in wp-config.php.
update_option('siteurl', 'http://your.domain.name/the/path' );
update_option('home', 'http://your.domain.name/the/path' );