why does session_start() change outputting? weird..

[sfullman]

Hi there, below is some code in an iframe which simulates running a process like emailing and updating. It works fine:

if($submode=='sendInvitations'){
	echo 'processing started..';
	for($i=1; $i<=100000; $i++){
		echo '.';
		if(!fmod($i,50)){
			//update the HTML progress meter every 50 records
			echo '<br />';
			?><script language="javascript" type="text/javascript">
			window.parent.setMeter(<?php echo $i?>,100000);
			</script><?php
			sleep(1);
		}
	}
}

Basically, the page continues to flush to the browser, and javascripts each run as they flush out, for a very user-friendly effect.

HOWEVER, and this is my question, I have to do session_start() on this document. But when I do, no output is sent until the page fully loads, basically making the status meter useless.

WHY DOES session_start() BUFFER PAGE OUTPUT LIKE THIS? HOW CAN i CHANGE THIS????

THANKS,
Samuel