4.1.0 The shutdown functions are now called as a part of the request. In earlier versions under Apache, the registered shutdown functions were called after the request has been completed (including sending any output buffers), so it was not possible to send output to the browser using echo() or print(), or retrieve the contents of any output buffers using ob_get_contents(). Headers were also always already sent.
da come dice qua in 4.1 la funzioen veniva richiamata quando apache aveva finito di inviare tutti i dati al client... cioè quando il client aveva appunto finito di scaricare tutto

??

ma in effetti quello poteva essere anche il solo tempo di invio dal server di apache