register_taxonomy()

register_taxonomy( string $taxonomy, array|string $object_type, array|string $args = array() ): WP_Taxonomy|WP_Error

function register_taxonomy( $taxonomy, $object_type, $args = array() ) {
	global $wp_taxonomies;

	if ( ! is_array( $wp_taxonomies ) ) {
		$wp_taxonomies = array();
	}

	$args = wp_parse_args( $args );

	if ( empty( $taxonomy ) || strlen( $taxonomy ) > 32 ) {
		_doing_it_wrong( __FUNCTION__, __( 'Taxonomy names must be between 1 and 32 characters in length.' ), '4.2.0' );
		return new WP_Error( 'taxonomy_length_invalid', __( 'Taxonomy names must be between 1 and 32 characters in length.' ) );
	}

	$taxonomy_object = new WP_Taxonomy( $taxonomy, $object_type, $args );
	$taxonomy_object->add_rewrite_rules();

	$wp_taxonomies[ $taxonomy ] = $taxonomy_object;

	$taxonomy_object->add_hooks();

	// Add default term.
	if ( ! empty( $taxonomy_object->default_term ) ) {
		$term = term_exists( $taxonomy_object->default_term['name'], $taxonomy );
		if ( $term ) {
			update_option( 'default_term_' . $taxonomy_object->name, $term['term_id'] );
		} else {
			$term = wp_insert_term(
				$taxonomy_object->default_term['name'],
				$taxonomy,
				array(
					'slug'        => sanitize_title( $taxonomy_object->default_term['slug'] ),
					'description' => $taxonomy_object->default_term['description'],
				)
			);

			// Update `term_id` in options.
			if ( ! is_wp_error( $term ) ) {
				update_option( 'default_term_' . $taxonomy_object->name, $term['term_id'] );
			}
		}
	}

	/**
	 * Fires after a taxonomy is registered.
	 *
	 * @since 3.3.0
	 *
	 * @param string       $taxonomy    Taxonomy slug.
	 * @param array|string $object_type Object type or array of object types.
	 * @param array        $args        Array of taxonomy registration arguments.
	 */
	do_action( 'registered_taxonomy', $taxonomy, $object_type, (array) $taxonomy_object );

	/**
	 * Fires after a specific taxonomy is registered.
	 *
	 * The dynamic portion of the filter name, `$taxonomy`, refers to the taxonomy key.
	 *
	 * Possible hook names include:
	 *
	 *  - `registered_taxonomy_category`
	 *  - `registered_taxonomy_post_tag`
	 *
	 * @since 6.0.0
	 *
	 * @param string       $taxonomy    Taxonomy slug.
	 * @param array|string $object_type Object type or array of object types.
	 * @param array        $args        Array of taxonomy registration arguments.
	 */
	do_action( "registered_taxonomy_{$taxonomy}", $taxonomy, $object_type, (array) $taxonomy_object );

	return $taxonomy_object;
}