output_handler in php.ini: how to use custom function

I'd like ot set the output_handler in php.ini to be a custom function I will write. Is this possible. The documentation say how to redirect to a function that is not part of the PHP core ... Basically I want to have all the output be sent to my custom function, which will do something to it, before being sent out to the browser. Since I will be doing this on all my pages I don't want to hard-code a call to ob_start() on each and every page. I am hoping I can do this with a setting in the php.ini file. Any suggestions? thanks, Jean-Christian Imbeault