register_taxonomy

register_taxonomy()

In order to have a taxonomy appear in the URL hierarchy of the relevant CPT, you can rewrite the taxonomy slug to contain the CPT’s slug. But you must register the CPT after registering the taxonomy, otherwise the rewrite will not work, i.e. in this case, book CPT must be registered after genre taxonomy.

/**
 * Create two taxonomies, genres and writers for the post type "book".
 *
 * @see register_post_type() for registering custom post types.
 */
function wpdocs_create_book_taxonomies() {
	// Add new taxonomy, make it hierarchical (like categories)
	$labels = array();
	$args = array(
		...,
		'labels'            => $labels,
		...,
	);

	register_taxonomy( 'genre', array( 'book' ), $args );

	unset( $args );
	unset( $labels );

	// Add new taxonomy, NOT hierarchical (like tags)
	$labels = array();
	$args = array(
		...,
		'labels'                => $labels,
		...
	);

	register_taxonomy( 'writer', 'book', $args );
}
// hook into the init action and call create_book_taxonomies when it fires
add_action( 'init', 'wpdocs_create_book_taxonomies', 0 );