Change logged in user of the active connection ()
resource fbsql_change_user ( string user, string password [, string database [, resource link_identifier ]] )


fbsql_change_user() changes the logged in user of the current active connection, or the connection given by the optional parameter link_identifier. If a database is specified, this will default or current database after the user has been changed. If the new user and password authorization fails, the current connected user stays active.