Description:
------------
A change was made to the PHP7 session handler such that PS_READ_FUNC implementations must return SUCCESS + ZSTR_EMPTY_ALLOC() for non-existent session IDs.
See related bug https://bugs.php.net/bug.php?id=71187 

Test script:
---------------
<?php
session_start();

$old_sessionid = session_id();

session_regenerate_id();

$new_sessionid = session_id();

echo "Old Session: $old_sessionid<br />";
echo "New Session: $new_sessionid<br />";

print_r($_SESSION);
?> 

Expected result:
----------------
session_regenerate_id shouldn't fail for empty/non-existent session IDs.

Actual result:
--------------
[31-Jan-2016 16:45:51 Europe/London] PHP Catchable fatal error:  session_regenerate_id(): Failed to create(read) session ID: wincache (path: C:\Windows\Temp) in [file location]