The Output Control functions allow you to control when output is
sent from the script. This can be useful in several different
situations, especially if you need to send headers to the browser
after your script has began outputting data. The Output Control
functions do not affect headers sent using
header() or setcookie(),
only functions such as echo() and data between
blocks of PHP code.
When upgrading from PHP 4.1.x (and 4.2.x) to 4.3.x due to a bug in
earlier versions you must ensure that
implict_flush is OFF in
your php.ini, otherwise any output with
ob_start() will not be hidden from output.