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 begun 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.
Nota:
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 implicit_flush is OFF in your php.ini, otherwise any output with ob_start() will not be hidden from output.