wp_insert_term

	 *
	 * The `$taxonomies` parameter passed to this filter is formatted as a SQL fragment. The
	 * {@see 'get_object_terms'} filter is recommended as an alternative.
	 *
	 * @since 2.8.0
	 *
	 * @param array    $terms      Array of terms for the given object or objects.
	 * @param int[]    $object_ids Array of object IDs for which terms were retrieved.
	 * @param string[] $taxonomies Array of taxonomy names from which terms were retrieved.
	 * @param array    $args       Array of arguments for retrieving terms for the given
	 *                             object(s). See wp_get_object_terms() for details.
	 */
	return apply_filters( 'wp_get_object_terms', $terms, $object_ids, $taxonomies, $args );
}

/**
 * Add a new term to the database.
 *
 * A non-existent term is inserted in the following sequence:
 * 1. The term is added to the term table, then related to the taxonomy.
 * 2. If everything is correct, several actions are fired.
 * 3. The 'term_id_filter' is evaluated.
 * 4. The term cache is cleaned.
 * 5. Several more actions are fired.
 * 6. An array is returned containing the `term_id` and `term_taxonomy_id`.
 *
 * If the 'slug' argument is not empty, then it is checked to see if the term
 * is invalid. If it is not a valid, existing term, it is added and the term_id
 * is given.
 *
 * If the taxonomy is hierarchical, and the 'parent' argument is not empty,
 * the term is inserted and the term_id will be given.
 *
 * Error handling:
 * If `$taxonomy` does not exist or `$term` is empty,
 * a WP_Error object will be returned.
 *
 * If the term already exists on the same hierarchical level,
 * or the term slug and name are not unique, a WP_Error object will be returned.
 *
 * @global wpdb $wpdb WordPress database abstraction object.
 *
 * @since 2.3.0
 *
 * @param string       $term     The term name to add.
 * @param string       $taxonomy The taxonomy to which to add the term.
 * @param array|string $args {
 *     Optional. Array or string of arguments for inserting a term.
 *
 *     @type string $alias_of    Slug of the term to make this term an alias of.
 *                               Default empty string. Accepts a term slug.
 *     @type string $description The term description. Default empty string.
 *     @type int    $parent      The id of the parent term. Default 0.
 *     @type string $slug        The term slug to use. Default empty string.
 * }
 * @return array|WP_Error An array containing the `term_id` and `term_taxonomy_id`,
 *                        WP_Error otherwise.
 */
function wp_insert_term( $term, $taxonomy, $args = array() ) {
	global $wpdb;

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

	/**
	 * Filters a term before it is sanitized and inserted into the database.
	 *
	 * @since 3.0.0
	 *
	 * @param string|WP_Error $term     The term name to add, or a WP_Error object if there's an error.
	 * @param string          $taxonomy Taxonomy slug.
	 */
	$term = apply_filters( 'pre_insert_term', $term, $taxonomy );

	if ( is_wp_error( $term ) ) {
		return $term;
	}

	if ( is_int( $term ) && 0 === $term ) {
		return new WP_Error( 'invalid_term_id', __( 'Invalid term ID.' ) );
	}

	if ( '' === trim( $term ) ) {
		return new WP_Error( 'empty_term_name', __( 'A name is required for this term.' ) );
	}

	$defaults = array(
		'alias_of'    => '',
		'description' => '',
		'parent'      => 0,
		'slug'        => '',
	);
	$args     = wp_parse_args( $args, $defaults );

	if ( $args['parent'] > 0 && ! term_exists( (int) $args['parent'] ) ) {
		return new WP_Error( 'missing_parent', __( 'Parent term does not exist.' ) );
	}

	$args['name']     = $term;
	$args['taxonomy'] = $taxonomy;

	// Coerce null description to strings, to avoid database errors.
	$args['description'] = (string) $args['description'];

	$args = sanitize_term( $args, $taxonomy, 'db' );

	// expected_slashed ($name)
	$name        = wp_unslash( $args['name'] );
	$description = wp_unslash( $args['description'] );
	$parent      = (int) $args['parent'];

	$slug_provided = ! empty( $args['slug'] );
	if ( ! $slug_provided ) {
		$slug = sanitize_title( $name );
	} else {
		$slug = $args['slug'];
	}

	$term_group = 0;
	if ( $args['alias_of'] ) {
		$alias = get_term_by( 'slug', $args['alias_of'], $taxonomy );
		if ( ! empty( $alias->term_group ) ) {
			// The alias we want is already in a group, so let's use that one.
			$term_group = $alias->term_group;
		} elseif ( ! empty( $alias->term_id ) ) {
			/*
			 * The alias is not in a group, so we create a new one
			 * and add the alias to it.
			 */
			$term_group = $wpdb->get_var( "SELECT MAX(term_group) FROM $wpdb->terms" ) + 1;

			wp_update_term(
				$alias->term_id,
				$taxonomy,
				array(
					'term_group' => $term_group,
				)
			);
		}
	}

	/*
	 * Prevent the creation of terms with duplicate names at the same level of a taxonomy hierarchy,
	 * unless a unique slug has been explicitly provided.
	 */
	$name_matches = get_terms(
		array(
			'taxonomy'               => $taxonomy,
			'name'                   => $name,
			'hide_empty'             => false,
			'parent'                 => $args['parent'],
			'update_term_meta_cache' => false,
		)
	);

	/*
	 * The `name` match in `get_terms()` doesn't differentiate accented characters,
	 * so we do a stricter comparison here.
	 */
	$name_match = null;
	if ( $name_matches ) {
		foreach ( $name_matches as $_match ) {
			if ( strtolower( $name ) === strtolower( $_match->name ) ) {
				$name_match = $_match;
				break;
			}
		}
	}

	if ( $name_match ) {
		$slug_match = get_term_by( 'slug', $slug, $taxonomy );
		if ( ! $slug_provided || $name_match->slug === $slug || $slug_match ) {
			if ( is_taxonomy_hierarchical( $taxonomy ) ) {
				$siblings = get_terms(
					array(
						'taxonomy'               => $taxonomy,
						'get'                    => 'all',
						'parent'                 => $parent,
						'update_term_meta_cache' => false,
					)
				);

				$existing_term = null;
				$sibling_names = wp_list_pluck( $siblings, 'name' );
				$sibling_slugs = wp_list_pluck( $siblings, 'slug' );

				if ( ( ! $slug_provided || $name_match->slug === $slug ) && in_array( $name, $sibling_names, true ) ) {
					$existing_term = $name_match;
				} elseif ( $slug_match && in_array( $slug, $sibling_slugs, true ) ) {
					$existing_term = $slug_match;
				}

				if ( $existing_term ) {
					return new WP_Error( 'term_exists', __( 'A term with the name provided already exists with this parent.' ), $existing_term->term_id );
				}
			} else {
				return new WP_Error( 'term_exists', __( 'A term with the name provided already exists in this taxonomy.' ), $name_match->term_id );
			}
		}
	}

	$slug = wp_unique_term_slug( $slug, (object) $args );

	$data = compact( 'name', 'slug', 'term_group' );

	/**
	 * Filters term data before it is inserted into the database.
	 *
	 * @since 4.7.0
	 *
	 * @param array  $data     Term data to be inserted.
	 * @param string $taxonomy Taxonomy slug.
	 * @param array  $args     Arguments passed to wp_insert_term().
	 */
	$data = apply_filters( 'wp_insert_term_data', $data, $taxonomy, $args );

	if ( false === $wpdb->insert( $wpdb->terms, $data ) ) {
		return new WP_Error( 'db_insert_error', __( 'Could not insert term into the database.' ), $wpdb->last_error );
	}

	$term_id = (int) $wpdb->insert_id;

	// Seems unreachable. However, is used in the case that a term name is provided, which sanitizes to an empty string.
	if ( empty( $slug ) ) {
		$slug = sanitize_title( $slug, $term_id );

		/** This action is documented in wp-includes/taxonomy.php */
		do_action( 'edit_terms', $term_id, $taxonomy );
		$wpdb->update( $wpdb->terms, compact( 'slug' ), compact( 'term_id' ) );

		/** This action is documented in wp-includes/taxonomy.php */
		do_action( 'edited_terms', $term_id, $taxonomy );
	}

	$tt_id = $wpdb->get_var( $wpdb->prepare( "SELECT tt.term_taxonomy_id FROM $wpdb->term_taxonomy AS tt INNER JOIN $wpdb->terms AS t ON tt.term_id = t.term_id WHERE tt.taxonomy = %s AND t.term_id = %d", $taxonomy, $term_id ) );

	if ( ! empty( $tt_id ) ) {
		return array(
			'term_id'          => $term_id,
			'term_taxonomy_id' => $tt_id,
		);
	}

	if ( false === $wpdb->insert( $wpdb->term_taxonomy, compact( 'term_id', 'taxonomy', 'description', 'parent' ) + array( 'count' => 0 ) ) ) {
		return new WP_Error( 'db_insert_error', __( 'Could not insert term taxonomy into the database.' ), $wpdb->last_error );
	}

	$tt_id = (int) $wpdb->insert_id;

	/*
	 * Sanity check: if we just created a term with the same parent + taxonomy + slug but a higher term_id than
	 * an existing term, then we have unwittingly created a duplicate term. Delete the dupe, and use the term_id
	 * and term_taxonomy_id of the older term instead. Then return out of the function so that the "create" hooks
	 * are not fired.
	 */
	$duplicate_term = $wpdb->get_row( $wpdb->prepare( "SELECT t.term_id, t.slug, tt.term_taxonomy_id, tt.taxonomy FROM $wpdb->terms t INNER JOIN $wpdb->term_taxonomy tt ON ( tt.term_id = t.term_id ) WHERE t.slug = %s AND tt.parent = %d AND tt.taxonomy = %s AND t.term_id < %d AND tt.term_taxonomy_id != %d", $slug, $parent, $taxonomy, $term_id, $tt_id ) );

	/**
	 * Filters the duplicate term check that takes place during term creation.
	 *
	 * Term parent+taxonomy+slug combinations are meant to be unique, and wp_insert_term()
	 * performs a last-minute confirmation of this uniqueness before allowing a new term
	 * to be created. Plugins with different uniqueness requirements may use this filter
	 * to bypass or modify the duplicate-term check.
	 *
	 * @since 5.1.0
	 *
	 * @param object $duplicate_term Duplicate term row from terms table, if found.
	 * @param string $term           Term being inserted.
	 * @param string $taxonomy       Taxonomy name.
	 * @param array  $args           Term arguments passed to the function.
	 * @param int    $tt_id          term_taxonomy_id for the newly created term.
	 */
	$duplicate_term = apply_filters( 'wp_insert_term_duplicate_term_check', $duplicate_term, $term, $taxonomy, $args, $tt_id );

	if ( $duplicate_term ) {
		$wpdb->delete( $wpdb->terms, array( 'term_id' => $term_id ) );
		$wpdb->delete( $wpdb->term_taxonomy, array( 'term_taxonomy_id' => $tt_id ) );

		$term_id = (int) $duplicate_term->term_id;
		$tt_id   = (int) $duplicate_term->term_taxonomy_id;

		clean_term_cache( $term_id, $taxonomy );
		return array(
			'term_id'          => $term_id,
			'term_taxonomy_id' => $tt_id,
		);
	}