WordPress is fast, lightweight, and easy to use. To ensure it stays that way, the Core Team thinks carefully about adding functionality to the core WordPress code. Still, users often find the need to graft additional functionality into WordPress to meet their needs. This section of the Codex offers guidelines and references for anyone wishing to modify, extend, or contribute to WordPress. Plugin Development Plugin Developer Handbook - The best starting place for learning about how to develop plugins WordPress Coding Standards - General information about coding standards for WordPress development Debugging in WordPress - Reference and guide for using the built-in debugging system in WordPress. Data Validation - A must-read for WordPress plugin authors. Describes the functions used by WordPress to validate and sanitize data. Plugin authors should be familiar with these functions and ideas. Plugin Submission and Promotion - Once you have written your plugin, here are some hints on distributing it widely Migrating Plugins and Themes - Contains information on how to upgrade your Plugin so it will work from version to version of WordPress Function Reference - Complete PHP function reference for WordPress Global Variables - A list of all global variables created by WordPress Post Types - Creating new types of posts other than the posts that display on the main loop. Taxonomies - Creating new types of taxonomies other than the built-in ones. Reserved Terms - A list of reserved terms in WordPress. APIs Plugin API - Hooks, Actions, and Filters to use in your Plugins (version 2.1; has links to older version articles) Shortcode API - A tutorial and reference for the shortcode API (new in version 2.5) Dashboard Widgets API - A reference with examples for adding new widgets to the admin dashboard. Settings API - A reference with examples for adding new settings to existing settings screens. Options API - Details about the generic option storage system. Transients API - Details about the temporary/time-constrained data storage system. Widgets API - A reference with examples for creating widgets for use in sidebars. Quicktags API - A reference for adding buttons to the HTML editor. Rewrite API - Details about the URL rewriting API. Theme Customization API - Details about the API for Theme Customization screen. Filesystem API - Reference for reading and writing local files to the filesystem to be done securely, on a variety of host types.