Learning PHP for WordPress
- PHP for Beginners: Starting on Backend WordPress Development – WP Shout
- PHP 101: Gentle Introduction to WordPress Programming – Know The Code
- WordPress Template Tags: How They Show Post Content & More (eg: the_title vs get_the_title) – WP Shout
Intermediate
- WP_Query Arguments – Bill Erickson
- Customizing the WordPress Query with pre_get_posts – Bill Erickson
- Custom WordPress Queries – Bill Erickson
- WP_Query – WordPress Code Reference