_Note: While I filed this under 3.1.7-pl1, the same issue existed in 3.0.
_
So, running phpBB on HHVM results in wrong URL paths. For example, when trying to change profile info, instead of being redirected to example.com/ucp.php, I get redirected to example.com/home/<user>/www/ucp.php.

I have forced server settings in the ACP, but no luck.

Temporarily I have patched this with the following code:

Find and comment in session.php:

$page_dir = str_repeat('../', sizeof($root_dirs)) . implode('/', $page_dirs);"

Add:

$page_dir = str_replace(phpbb_realpath($root_path), '', phpbb_realpath('./'));