[CATEGORY & TAXONOMY] sử dụng get_categories để lấy bài viết thuộc category, taxonomy (ok)

https://developer.wordpress.org/reference/functions/get_categories/

C:\xampp\htdocs\wordpress\wp-content\themes\twentytwentyone\functions.php

// ===========
add_action('init', 'my_book_cpt');
function my_book_cpt() {
  $labels = array(
    'name'               => _x('Books', 'post type general name', 'your-plugin-textdomain'),
    'singular_name'      => _x('Book', 'post type singular name', 'your-plugin-textdomain'),
    'menu_name'          => _x('Books', 'admin menu', 'your-plugin-textdomain'),
    'name_admin_bar'     => _x('Book', 'add new on admin bar', 'your-plugin-textdomain'),
    'add_new'            => _x('Add New', 'book', 'your-plugin-textdomain'),
    'add_new_item'       => __('Add New Book', 'your-plugin-textdomain'),
    'new_item'           => __('New Book', 'your-plugin-textdomain'),
    'edit_item'          => __('Edit Book', 'your-plugin-textdomain'),
    'view_item'          => __('View Book', 'your-plugin-textdomain'),
    'all_items'          => __('All Books', 'your-plugin-textdomain'),
    'search_items'       => __('Search Books', 'your-plugin-textdomain'),
    'parent_item_colon'  => __('Parent Books:', 'your-plugin-textdomain'),
    'not_found'          => __('No books found.', 'your-plugin-textdomain'),
    'not_found_in_trash' => __('No books found in Trash.', 'your-plugin-textdomain'),
  );
  $args = array(
    'labels'             => $labels,
    'description'        => __('Description.', 'your-plugin-textdomain'),
    'public'             => true,
    'publicly_queryable' => true,
    'show_ui'            => true,
    'show_in_menu'       => true,
    'query_var'          => true,
    'rewrite'            => array('slug' => 'book'),
    'capability_type'    => 'post',
    'has_archive'        => true,
    'hierarchical'       => false,
    'menu_position'      => null,
    'show_in_rest'       => true,
    'rest_base'          => 'books',
    'supports'           => array('title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments'),
  );
  register_post_type('book', $args);
  flush_rewrite_rules(true);
}
// ===========
add_action('init', 'my_book_taxonomy', 30);
function my_book_taxonomy() {
  $labels = array(
    'name'              => _x('Genres', 'taxonomy general name'),
    'singular_name'     => _x('Genre', 'taxonomy singular name'),
    'search_items'      => __('Search Genres'),
    'all_items'         => __('All Genres'),
    'parent_item'       => __('Parent Genre'),
    'parent_item_colon' => __('Parent Genre:'),
    'edit_item'         => __('Edit Genre'),
    'update_item'       => __('Update Genre'),
    'add_new_item'      => __('Add New Genre'),
    'new_item_name'     => __('New Genre Name'),
    'menu_name'         => __('Genre'),
  );
  $args = array(
    'hierarchical'      => true,
    'labels'            => $labels,
    'show_ui'           => true,
    'show_admin_column' => true,
    'query_var'         => true,
    'rewrite'           => array('slug' => 'genre'),
    'show_in_rest'      => true,
    'rest_base'         => 'genre',
  );
  register_taxonomy('genre', array('book'), $args);
  flush_rewrite_rules(true);
}

Ví dụ 1: Sử dụng mặc định 'taxonomy' => 'category'

C:\xampp\htdocs\wordpress\wp-content\themes\twentytwentyone\header.php

<?php  
				$categories = get_categories( 
					array(
						'taxonomy' => 'category',
				    'orderby' => 'name',
				    'order'   => 'ASC'
					)
				);
				echo '<pre>';
					var_export($categories);
				echo '</pre>';
			?>

Kết quả:

Ví dụ 2: Sử dụng 'taxonomy' => 'genre'

C:\xampp\htdocs\wordpress\wp-content\themes\twentytwentyone\header.php

<?php  
	$categories = get_categories( 
		array(
			'taxonomy' => 'genre',
	    'orderby' => 'name',
	    'order'   => 'ASC'
		)
	);
	echo '<pre>';
		var_export($categories);
	echo '</pre>';
?>

Kết quả:

Ví dụ 3: Sử dụng hỗn hợp 'taxonomy' => 'genre', 'taxonomy' => 'category'

C:\xampp\htdocs\wordpress\wp-content\themes\twentytwentyone\header.php

<?php  
				$categories = get_categories( 
					array(
						'taxonomy' => array('genre','category'),
				    'orderby' => 'name',
				    'order'   => 'ASC'
					)
				);
				echo '<pre>';
					var_export($categories);
				echo '</pre>';
			?>