Description:
------------
Found this behavior on a productive System:
User calls URI with empty SID parameter like this: 
test.php?mySID=
php / memcache spits out warnings and session functionality is not available on this page (write doesn't work).

Standard PHP files session handler handles this error by generating a new id and uses the new one to write session data and sends a cookie with the new id.

memcache session handler used to throw a ugly warning. Since 3.0.4 it just ignores the problem. Leaves developers alone with probably messed up pages.

Instead of ignoring the issue memcache should generate a new id and use it just like the standard files handler would do.


Versions:
 PHP: from 5.2.6 to 5.3.1 stable
 memcache: all from 2.2.5 to 3.0.4

 tested on lighttpd with php-cgi

php.ini changes:
 session.save_handler = memcache
 session.save_path = "tcp://localhost:11211"
 session.use_only_cookies = 0


Reproduce code:
---------------
<?
  session_name('mySID');

  session_start();
  echo session_name()."=".session_id()."<br>\n";

  $_SESSION['count']++;
  var_dump($_SESSION);



workaround: 
<?
  session_name('mySID');

  if ($_GET[session_name()] == "") unset($_GET[session_name()]);

  session_start();
  echo session_name()."=".session_id()."<br>\n";

  $_SESSION['count']++;
  var_dump($_SESSION);

Expected result:
----------------
A new generated Session_id on every reguest for 'test.php?mySID='
if client disabled cookies.

Actual result:
--------------
Empty Session_id and no write of session data.
On older memcache versions (<3.0.3) PHP Warnings and Errors like: 

Warning: Unknown: Failed to write session data (memcache). Please verify that the current setting of session.save_path is correct (tcp://localhost:11211) in Unknown on line 0

Warning: session_start() [function.session-start]: Key cannot be empty in /var/www/index.php on line 4