Session->userdata

$form_token = uniqid();           
$this->session->set_userdata("form_token",  $form_token);

//if call session using single quotes will generate wrong output
$this->session->userdata('form_token');

//if call session using double quotes will generate right output.
$this->session->userdata("form_token");
why its so??

It isn't so, you're doing something else not shown in this example.