Description:
------------
If I call preg_split with a string such as ",,foo,,bar,," I expect to get five empty strings back in the returned array, but I only get back two - the first and the last.

Reproduce code:
---------------
$tokens = preg_split("/[,]+/", ",,foo,,bar,,");

Expected result:
----------------
I expect $tokens to contain the following array:

array(12) {
  [0]=>  string(0) ""
  [1]=>  string(0) ""
  [2]=>  string(3) "foo"
  [3]=>  string(0) ""
  [4]=>  string(3) "bar"
  [5]=>  string(0) ""
  [6]=>  string(0) ""
}


Actual result:
--------------
array(12) {
  [0]=>  string(0) ""
  [1]=>  string(3) "foo"
  [2]=>  string(3) "bar"
  [3]=>  string(0) ""
}