Register and use custom WordPress post statuses in your plugin
Register custom post statuses in WordPress with register_post_status and inject them into the classic editor dropdown with JavaScript.
Add a custom meta box to the WordPress post editor with secure save hand ...
Register a custom meta box on WordPress posts and pages with wp_nonce_field CSRF protection and sanitised save_post handling.
Add custom admin notices in WordPress with PHP
Display success, warning and error admin notices in WordPress with persistent AJAX dismissal stored in user meta.
Build a custom WordPress widget with WP Widget
Extend WP_Widget to create a configurable sidebar widget with an admin settings form in a classic WordPress theme.
WordPress conditional tags explained with practical examples
Master is_single, is_page, is_archive and other WordPress conditional tags to scope your code correctly.