Using "OFF" or no value on output_buffering will disable header modifications, like redirects or content-type or content-disposition resulting in the error we commonly attribute to output before header modifications:
Warning: Cannot modify header information - headers already sent by (output started at C:\PATH\filename.php:1) C:\PATH\filename.php on line 1
Example code with output_buffering = OFF which results in this behavior. Changing it to "ON" or giving it a value will likely cause normal behavior.
<?php header("Location: http://www.php.net"); ?>
<?php header("Content-Type: text/Calendar"); ?>
<?php header("Content-Disposition: inline; filename=appointment.ics"); ?>