On Ubuntu or Debian machines, if session.save_path is not set, then session files are saved in /var/lib/php5 by default.

If you check the hhvm logs in /var/log/hhvm/error.log there is an error

\nFatal error: Failed to initialize storage module: user (path: /var/lib/php5) in /var/www/html/commerce-demo/includes/session.inc on line 287

Drupal/PHP fails to create a session in the drupal_session_start() function because the default location to store the session files /var/lib/php5 does not exist or is not known within our setup with hhvm.

In my Drupal settings.php file I added to the bottom

ini_set("session.save_path", "");

This resets the path to "" (an empty string), by default this evaluates to the temp folder of the system. This helped me: http://stackoverflow.com/questions/4927850/location-for-session-files-in-apache-php

To find the current session save path, you can use session_save_path() (Get and/or set the current session save path).

Why drupal failed to create only the sessions related to the checkout page is not clear to me, but now it works ;)

I found out that there is a difference between how Zend and HHVM interprete the php key() function.

In the Drupal commerce_checkout module there is a function that returns the first key of an array by using the php key() function (commerce_checkout.module, line 480).

/**
 * Returns the page ID of the first checkout page sorted by weight.
 */
function commerce_checkout_first_checkout_page() {
  return key(commerce_checkout_pages());
}

On the default Zend rendering engine this returned the desired result of the first array key. But in HHVM this returned nothing. I believe this is because the pointers of the array were not flushed by HHVM (maybe for performance reasons), and they were flushed in the Zend engine.

I don't think it was a good idea of the creaters of this module to use the key() function to get the first key of the array anyway

I now rewrote this function to this:

/**
 * Returns the page ID of the first checkout page sorted by weight. 
 */
function commerce_checkout_first_checkout_page() {
 return reset(array_keys(commerce_checkout_pages()));
}

Getting the keys of the array first and then with reset() rewind the array's internal pointer to the first element and return the value of that first array element. Of course the key() function did the job on the standard Zend engine, but for HHVM this didn't.

I will report this issue to the maintenance crew of the drupal commerce module, and hopefully it will be commited into the next release. I don't think there is anything wrong with the way HHVM interprets the key function (if I am right in thinking this is the root of the problem). I think the key() function just shouldn't be used like this in this situation.