Codeigniter Form POST Question

On my login controller I redirect to my dashboard if form success. But when I redirect it to my dashboard controller I cannot get post data

I use var dump on my login controller

var_dump($_POST);
exit;

And I can see post data then


I try to use <?php echo $this->input->post('username');?> on dashboard after forum success 

Most of the time I use $this->session->userdata('username') but would like to know why after been redirected can not echo post username

I know its a basic php question but would be good to know.

There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!