WordPress: Get term IDs, creates terms if they don't exist

get-term-ids.php

/**
 * Gets the term IDs for the term names. Creates the terms if they do not exist.
 * TODO update the project_name_term_id_created filter when you copy/paste this function.
 *
 * @param  string $term_names The term name (or names separated with pipes).
 * @param  string $taxonomy   The taxonomy.
 * @param  string $by         Get term by 'name' or 'slug'?
 * @param  bool   $create     Create the term if it doesn't exist?
 * @return array
 */
function get_term_ids( $term_names, $taxonomy, $by = 'name', $create = true ) {

	$term_ids = [];

	if ( ! taxonomy_exists( $taxonomy ) ) {
		return $term_ids;
	}

	foreach ( explode( '|', $term_names ) as $term_name ) {

		$term_name = trim( $term_name );

		if ( empty( $term_name ) ) {
			continue;
		}

		$term_data = false;

		// Only allow lookups by name or slug.
		if ( 'name' !== $by && 'slug' !== $by ) {
			$by = 'name';
		}

		$existing_term = get_term_by( $by, $term_name, $taxonomy );

		if ( $existing_term instanceof \WP_Term ) {
			$term_data = [ 'term_id' => $existing_term->term_id ];
		}

		if ( ! is_array( $term_data ) ) {
			if ( $create ) {
				$term_data = wp_insert_term( $term_name, $taxonomy );

				if ( is_array( $term_data ) ) {

					// Work around an existing bug in https://github.com/Automattic/vip-go-mu-plugins/issues/1832.
					$cache_key   = $term_name . '|' . $taxonomy;
					$cache_group = 'term_exists';
					wp_cache_delete( $cache_key, $cache_group );

					do_action( 'project_name_term_id_created', $term_data['term_id'] );
				}
			}
		}

		if ( is_array( $term_data ) ) {
			$term_ids[] = $term_data['term_id'];
		}
	}

	return array_map( 'absint', $term_ids );
}