Request: function as register_shutdown_function, but as it was before PHP 4.1.0

I'm asking for a function as register_shutdown_function used to be, when it executed after client disconnects. Reason: Get faster responses to the user when some considerably 'slow' process might be executed in background, after processes. I know it could be done by some custom function, as well as it's possible through Output Control Buffers Functions, but it's more code to write, more commands to be processed by server and it'd 'conflict' with output_buffering setting from php.ini so you'd also need to manage it. Also, the current function could be simulated easier than the old one, but it's the one chosen. It can easily be done by you, as it's just copy & paste, so I hope it's implemented. Thanks in advance.