Codex

Below are articles related to WordPress in order of importance for newcomers to learn.

Required

• Theme file hierarchy (template)
• 3 ways to create a page template
• Theme file functions.php
• What is the “the loop” cycle in WordPress
• WordPress child themes
• How hooks work in WordPress (filters and actions)
• Auto-updates in WordPress
• Translations in WordPress (localization)
• Debugging in WordPress

Types of data

• Posts in WordPress
• Taxonomies in WordPress
• Metadata in WordPress
• Comments in WordPress

For developers

• 3 ways to build loops in WordPress
• Ajax in WordPress
• WordPress HTTP API
• Data validation in WordPress
• Must-Use plugins
• Settings API (options)
• WordPress constants
• Global variables in WordPress
• SHORTINIT constant
• Busy query variable names (blacklist)
• PHP code standards
• Object cache in WordPress
• WP-Cron (task scheduler)

It’s also good to know

• Feeds (RSS) in WordPress
• WordPress Quicktags API
• ThickBox API: modal window in the admin panel
• Heartbeat API
• How notifications work (pings, trackbacks) in WordPress

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Official page of the tutorial: https://developer.wordpress.org/apis/