Best error handling

Hi. I have a kind of webserver that certain applications communicate with. The problem is that the script (named peer.php) must output data as xml. Thats ok (thanks to XMLWriter). The problem is that when some error occurs (those note catchable by set_error_handler) I have no control on data being outputed. I would like to output xml even on fatal errors (in that case, telling that something gone wrong). I tried using register_shutdown_function but it is crashing (as the nature of fatal errors that make the envirionment unstable). Any idea? Thanks! -- Davi R. Tavares