Hiii
ob_start -- Turn on output buffering
That means no output is sent from the script except from the headers. Instead it's stored in the internal buffer.

Heres the reference at php.net: http://www.php.net/ob_start

ob_start() is part of the output control functions in PHP. See: http://www.php.net/manual/en/ref.outcontrol.php

Calling ob_start() begins the buffer of output that is normally sent to HTTP (with PHP on a webserver).

Do to the HTTP specification, you cannot send any HTTP Headers after you have sent some HTTP Content. ob_start() allows you to have the HTTP Content placed in a buffer, so that you can send HTTP headers at any time. Then at the end of your PHP script, call ob_end_flush() which will flush the HTTP Content.

Edit: Looks like php_coder got to your question first.