fbsql_insert_id() returns the ID generated for an column defined as DEFAULT UNIQUE by the previous INSERT query using the given link_identifier. If link_identifier isn't specified, the last opened link is assumed.
fbsql_insert_id() returns 0 if the previous query does not generate an DEFAULT UNIQUE value. If you need to save the value for later, be sure to call fbsql_insert_id() immediately after the query that generates the value.
The value of the FrontBase SQL function fbsql_insert_id() always contains the most recently generated DEFAULT UNIQUE value, and is not reset between queries.