coolsix: ob_start() is just a walkaround, but in most cases, call your PHP Scripts and Sessions before any HTML.

Although ob_start() should fix it.

But
<?php
ob_flush(); //ob_flush first to flush previous cache
ob_start(); //Start a new Buffer
session_start(); //call the session
?>

For some reason, calling just flush or ob_flush or even both together did not get my output buffers flushed, and calling ob_end_flush by itself didn't work either but calling them all worked well. Here is my new output flushing function.

<?php
function flush_buffers(){
ob_end_flush();
ob_flush();
flush();
ob_start();
}
?>
http://php.net/manual/en/function.ob-flush.php

maekhel: make sure u are using session_start before any HTML request is sent

coolsix:

session_start() can be used after HTML as long as there is <?php ob_start(); ?> or <?php ob_flush(); ?> at the top of the script