Post type/taxonomy mirror.

init.php

<?php


$mirror = new PostTypeTaxonomyMirror( 'my_post_type', 'my_taxonomy' );
$mirror->init();

PostTypeTaxonomyMirror.php

<?php


class PostTypeTaxonomyMirror {


	private $post_type;
	private $taxonomy;


	/**
	 * @param $post_type
	 * @param $taxonomy
	 */
	public function __construct( $post_type, $taxonomy ) {
		$this->post_type = $post_type;
		$this->taxonomy = $taxonomy;
	}


	public function init() {
		$this->update_terms_on_post_update();
		$this->delete_terms_on_post_delete();
	}


	// NOTE: this doesn't appear to be relevant as the posts are 'updated' when they are created in the admin. This might be something we need to investigate for when creating posts via PHP
	//private function create_terms_on_post_create() {
	//	add_action( "save_post_{$this->post_type}", function ( $post_id, WP_Post $post, $updated ) {
	//		PDS_Debug::log( $updated );
	//		if ( ! $updated ) {
	//			$result = wp_insert_term( $post->post_name, $this->taxonomy, [ 'name' => $post->post_title ] );
	//			//wp_add_object_terms($post_id, $result->)
	//			PDS_Debug::log( $result );
	//		}
	//	}, 10, 3 );
	//}


	private function update_terms_on_post_update() {
		add_action( 'post_updated', function ( $post_id, WP_Post $after, WP_Post $before ) {

			if ( get_post_type( $post_id ) !== $this->post_type ) {
				return;
			}

			if ( $this->new_post_created( $after, $before ) ) {
				$this->create_new_term_for_post( $after );

			} elseif ( $this->post_has_changed( $before, $after ) ) {
				if ( $term = $this->get_existing_post_term( $before ) ) {
					$this->update_post_term( $term, $after );

				} else {
					$this->create_new_term_for_post( $after );
				}

			} elseif ( ! $term = $this->get_existing_post_term( $after ) ) {
				$this->create_new_term_for_post( $after );
			}

		}, 10, 3 );
	}


	private function delete_terms_on_post_delete() {
		add_action( 'delete_post', function ( $post_id ) {

			if ( get_post_type( $post_id ) !== $this->post_type ) {
				return;
			}

			if ( $post = get_post( $post_id ) and $term = $this->get_existing_post_term( $post ) ) {
				wp_delete_term( $term->term_id, $this->taxonomy );
			}

		} );
	}


	private function new_post_created( WP_Post $after, WP_Post $before ) {
		return $before and $before->post_status === 'auto-draft';
	}


	private function create_new_term_for_post( WP_Post $post ) {
		$result = wp_insert_term( $post->post_title, $this->taxonomy, [ 'slug' => $post->post_name ] );
		if ( ! is_wp_error( $result ) ) {
			wp_add_object_terms( $post->ID, $result['term_id'], $this->taxonomy );
		}
	}


	private function post_has_changed( WP_Post $after, WP_Post $before ) {
		return ! (
			$before->post_name === $after->post_name and
			$before->post_title === $after->post_title
		);
	}


	private function get_existing_post_term( WP_Post $post ) {
		$term = get_term_by( 'slug', $post->post_name, $this->taxonomy, OBJECT );

		return $term instanceof WP_Term
			? $term
			: false;
	}


	private function update_post_term( WP_Term $term, WP_Post $post ) {
		wp_update_term( $term->term_id, $this->taxonomy, [
			'slug' => $post->post_name,
			'name' => $post->post_title,
		] );
	}


}