As said in the summary, starting a session turns on some strange output-buffering, wich cannot be overrided per flush(). This happens since 4.2.0 and is still in the latest snapshot, until 4.1.2 it was okay.

Sample-Scipt:

<?php

session_start();
session_write_close();

set_time_limit (0);
$i = 0;
for(;;)
{
$time = time();
$newtime = date("H:i:s", $time);
printf("($newtime) - $i <br>");
$i++;
flush();
usleep(200000);
}
?>

To me, this is VERY important, because it makes session-based webchats nearly unusuable.