Description:
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When a read from the session backend fails it still initializes the session without data. This causes problems during either the closing of the session or the php_session_decode (depending on the serializer) with writing an empty session or destroying the session data respectively.

I choose E_ERROR to be in line with s_open and s_create_id function checks, but it can easily changed into E_WARNING to be more in line with the original comment and would cause a return FALSE, because PS(session_status) is not php_session_active.

I haven't looked into other versions, but I guess this applies to other versions as well.

Test script:
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Originally found with https://github.com/php-memcached-dev/php-memcached/ as a session backend.
The script below can be tested by setting up memcached and doing a
"add memc.sess.lock.<session_id> 0 0 1"  on a telnet session to memcached (make sure that memcached.session_locking is set, which it is by default).
The request will first hang and at the end reset the session content in memcached.

<?php
ini_set('session.save_handler', 'memcached');
ini_set('session.save_path', "127.0.0.01:11211");

session_start();
var_dump($_SESSION, session_id());
$_SESSION["a"] = "b";