wp_set_object_terms( int $object_id, string|int|array $terms, string $taxonomy, bool $append = false )

function wp_set_object_terms( $object_id, $terms, $taxonomy, $append = false ) {
	global $wpdb;

	$object_id = (int) $object_id;

	if ( ! taxonomy_exists( $taxonomy ) ) {
		return new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
	}

	if ( !is_array($terms) )
		$terms = array($terms);

	if ( ! $append )
		$old_tt_ids =  wp_get_object_terms($object_id, $taxonomy, array('fields' => 'tt_ids', 'orderby' => 'none'));
	else
		$old_tt_ids = array();

	$tt_ids = array();
	$term_ids = array();
	$new_tt_ids = array();

	foreach ( (array) $terms as $term) {
		if ( !strlen(trim($term)) )
			continue;

		if ( !$term_info = term_exists($term, $taxonomy) ) {
			// Skip if a non-existent term ID is passed.
			if ( is_int($term) )
				continue;
			$term_info = wp_insert_term($term, $taxonomy);
		}
		if ( is_wp_error($term_info) )
			return $term_info;
		$term_ids[] = $term_info['term_id'];
		$tt_id = $term_info['term_taxonomy_id'];
		$tt_ids[] = $tt_id;

		if ( $wpdb->get_var( $wpdb->prepare( "SELECT term_taxonomy_id FROM $wpdb->term_relationships WHERE object_id = %d AND term_taxonomy_id = %d", $object_id, $tt_id ) ) )
			continue;

		/**
		 * Fires immediately before an object-term relationship is added.
		 *
		 * @since WP-2.9.0
		 * @since WP-4.7.0 Added the `$taxonomy` parameter.
		 *
		 * @param int    $object_id Object ID.
		 * @param int    $tt_id     Term taxonomy ID.
		 * @param string $taxonomy  Taxonomy slug.
		 */
		do_action( 'add_term_relationship', $object_id, $tt_id, $taxonomy );
		$wpdb->insert( $wpdb->term_relationships, array( 'object_id' => $object_id, 'term_taxonomy_id' => $tt_id ) );

		/**
		 * Fires immediately after an object-term relationship is added.
		 *
		 * @since WP-2.9.0
		 * @since WP-4.7.0 Added the `$taxonomy` parameter.
		 *
		 * @param int    $object_id Object ID.
		 * @param int    $tt_id     Term taxonomy ID.
		 * @param string $taxonomy  Taxonomy slug.
		 */
		do_action( 'added_term_relationship', $object_id, $tt_id, $taxonomy );
		$new_tt_ids[] = $tt_id;
	}

	if ( $new_tt_ids )
		wp_update_term_count( $new_tt_ids, $taxonomy );

	if ( ! $append ) {
		$delete_tt_ids = array_diff( $old_tt_ids, $tt_ids );

		if ( $delete_tt_ids ) {
			$in_delete_tt_ids = "'" . implode( "', '", $delete_tt_ids ) . "'";
			$delete_term_ids = $wpdb->get_col( $wpdb->prepare( "SELECT tt.term_id FROM $wpdb->term_taxonomy AS tt WHERE tt.taxonomy = %s AND tt.term_taxonomy_id IN ($in_delete_tt_ids)", $taxonomy ) );
			$delete_term_ids = array_map( 'intval', $delete_term_ids );

			$remove = wp_remove_object_terms( $object_id, $delete_term_ids, $taxonomy );
			if ( is_wp_error( $remove ) ) {
				return $remove;
			}
		}
	}

	$t = get_taxonomy($taxonomy);
	if ( ! $append && isset($t->sort) && $t->sort ) {
		$values = array();
		$term_order = 0;
		$final_tt_ids = wp_get_object_terms($object_id, $taxonomy, array('fields' => 'tt_ids'));
		foreach ( $tt_ids as $tt_id )
			if ( in_array($tt_id, $final_tt_ids) )
				$values[] = $wpdb->prepare( "(%d, %d, %d)", $object_id, $tt_id, ++$term_order);
		if ( $values )
			if ( false === $wpdb->query( "INSERT INTO $wpdb->term_relationships (object_id, term_taxonomy_id, term_order) VALUES " . join( ',', $values ) . " ON DUPLICATE KEY UPDATE term_order = VALUES(term_order)" ) )
				return new WP_Error( 'db_insert_error', __( 'Could not insert term relationship into the database.' ), $wpdb->last_error );
	}

	wp_cache_delete( $object_id, $taxonomy . '_relationships' );
	wp_cache_delete( 'last_changed', 'terms' );

	/**
	 * Fires after an object's terms have been set.
	 *
	 * @since WP-2.8.0
	 *
	 * @param int    $object_id  Object ID.
	 * @param array  $terms      An array of object terms.
	 * @param array  $tt_ids     An array of term taxonomy IDs.
	 * @param string $taxonomy   Taxonomy slug.
	 * @param bool   $append     Whether to append new terms to the old terms.
	 * @param array  $old_tt_ids Old array of term taxonomy IDs.
	 */
	do_action( 'set_object_terms', $object_id, $terms, $tt_ids, $taxonomy, $append, $old_tt_ids );
	return $tt_ids;
}