wp_check_term_hierarchy_for_loops( int $parent, int $term_id, string $taxonomy )

Source

File: wp-includes/taxonomy.php

function wp_check_term_hierarchy_for_loops( $parent, $term_id, $taxonomy ) {
	// Nothing fancy here - bail.
	if ( ! $parent ) {
		return 0;
	}

	// Can't be its own parent.
	if ( $parent === $term_id ) {
		return 0;
	}

	// Now look for larger loops.
	$loop = wp_find_hierarchy_loop( 'wp_get_term_taxonomy_parent_id', $term_id, $parent, array( $taxonomy ) );
	if ( ! $loop ) {
		return $parent; // No loop.
	}

	// Setting $parent to the given value causes a loop.
	if ( isset( $loop[ $term_id ] ) ) {
		return 0;
	}

	// There's a loop, but it doesn't contain $term_id. Break the loop.
	foreach ( array_keys( $loop ) as $loop_member ) {
		wp_update_term( $loop_member, $taxonomy, array( 'parent' => 0 ) );
	}

	return $parent;
}