php – cannot output session variable in json response

This is probably really simple but it isn’t obvious to me right now why this isn’t working.

I am setting a session variable on the index.php file, putting the value into a hidden field within a form and then submitting the form via jQuery and ajax. All the other form data is there and the hidden input value is there. But if I just try to echo the actual session variable out I get nothing eg:

$response['message'] = $_SESSION['csrf_token']; // no output

$response['message'] = $_POST['token']; // outputs the session variable value stored in the hidden field

On index.php

<?php
session_start();
$_SESSION['csrf_token'] = uniqid('', true);
?>

 <input type="hidden" name="token" value="<?php echo $_SESSION['csrf_token']; ?>">

jQuery:

var form = $('#testForm').serialize();
            $.ajax({
                url: 'test',
                type: 'POST',
                dataType: 'json',
                data: form,
                beforeSend: function() {

                    // loading spinner etc.
                }
            })

session_start();
$response['message'] = $_SESSION['csrf_token'];
$response['message'] = $_POST['token'];

Tags: json, phpphp, session