会话从php页面丢失到codeigniter项目

I have a button that POST data to my site

<form action="https://www.mysite.co.uk/some/index.php" method="POST">
    <input type="hidden" name="userid" value="ZUpmenVaN0ZVTTBmejNGZGNwZGFha1NmR0tuSjdaT3VYdjV5cTF4WGtISzRvK0ptOC9vZmQyc3J3T3cwTmplbWZ3alhod0xMYUhlQ2xLSng4WWI4ZEE9PQ2">
    <input type="submit" value="Go" style="font-size:14px; padding:20px;">
</form>

I then turn it into usable session

mysite.co.uk/some/index.php

<?php session_start();
 <?php
    if (isset($_POST['userid'])) {
        $_SESSION['userid'] = $_POST['userid'];
        $userid = $_SESSION['userid'];
    }
    if (isset($_SESSION['userid'])) {
        echo "You are logged in and have access to these tests.";?>
code here
  <?php
    } else {
        header ('location: ../index.htm');
    }
    ?>

this all works well.

I now want to use this within a countersign project with the same website (this is where it goes wrong)

mysite.co.uk/some/tests/ETray/New/

 <?php
  $this->load->library('session');
  $this->session->set_userdata('user_id', $_POST['userid']);
  $userID = $this->session->userdata('user_id');
  echo "$userID";
  ?>

I get error

Message: Undefined index: userid

unless I post direct to this page like

<form action="https://www.mysite.co.uk/some/tests/ETray/New/" method="POST">
    <input type="hidden" name="userid" value="ZUpmenVaN0ZVTTBmejNGZGNwZGFha1NmR0tuSjdaT3VYdjV5cTF4WGtISzRvK0ptOC9vZmQyc3J3T3cwTmplbWZ3alhod0xMYUhlQ2xLSng4WWI4ZEE9PQ2">
    <input type="submit" value="Go" style="font-size:14px; padding:20px;">
</form>

Then no error and the echo prints out.

Question

how do I get this to echo out on this page with the first form not the second ?

Looks like that error Message: Undefined index: useridis due to the undefined index of $_POST['userid']

Another one, where does mysite.co.uk/some/tests/ETray/New/ points to?? Is that inside controller or where???