WordPress function to auto-populate a taxonomy with a custom post type's entries.

wp-autopopulate-taxonomy.php

<?php
/**
 * Register our custom taxonomy.
 *
 * @return void
 */
function gist_5f2e8bcd3217698fb4947e120f178686_custom_tax_init() {
	// Set some options for our new custom taxonomy.
	$args = array(
		'label'        => __( 'My Custom Taxonomy' ),
		'hierarchical' => true,
		'capabilities' => array(
			// Allow anyone editing posts to assign terms.
			'assign_terms' => 'edit_posts',
			// ...but only admins can change what was auto-generated.
			'edit_terms'   => 'administrator',
		),
	);

	// Create the custom taxonomy and attach it to a custom post type.
	register_taxonomy( 'my-taxonomy', 'post-type-A', $args );
}
add_action( 'init', 'gist_5f2e8bcd3217698fb4947e120f178686_custom_tax_init' );


/**
 * When a post is saved, check if a corresponding taxonomy term exists. If not, create one.
 *
 * Fires when a post is created or edited.
 *
 * @param int $post_id The ID of the post that's being saved.
 * @return void
 */
function gist_5f2e8bcd3217698fb4947e120f178686_update_custom_terms( $post_id ) {
	// Only update terms if it's a post-type-B post.
	if ( 'post-type-B' !== get_post_type( $post_id ) ) {
		return;
	}

	// Don't create or update terms for system-generated posts.
	if ( get_post_status( $post_id ) === 'auto-draft' ) {
		return;
	}

	// Grab the post title and slug to use as the new or updated term name and slug.
	$term_title = get_the_title( $post_id );
	$term_slug  = get_post( $post_id )->post_name;

	// Check if a corresponding term already exists by comparing the post ID to all existing term descriptions.
	$existing_terms = get_terms( 'my-taxonomy', array(
		'hide_empty' => false,
		'meta_key'   => 'associated_post-type-B',
		'meta_value' => $post_id,
	));

	if ( ! empty( $existing_terms[0] ) ) {
		// The term already exists, so update it and we're done.
		$term = $existing_terms[0];
		$return = wp_update_term(
			$term->term_id,
			'my-taxonomy',
			array(
				'name' => $term_title,
				'slug' => $term_slug,
			)
		);
		return; // Return early to prevent the next chunk from executing.
	}

	// If we didn't find a match above, this is a new post, so create a new term.
	$new_term = wp_insert_term(
		$term_title,
		'my-taxonomy',
		array(
			'slug' => $term_slug,
		)
	);
	if ( ! is_wp_error( $new_term ) && ! empty( $new_term['term_id'] ) ) {
		// Save the post ID as metadata for the term.
		add_term_meta( $new_term['term_id'], 'associated_post-type-B', $post_id, true );
	}
}
// Run the update function whenever a post is created or edited.
add_action( 'save_post', 'gist_5f2e8bcd3217698fb4947e120f178686_update_custom_terms' );