Description:
------------
Multiple calls to nextRowset() or closeCursor() (only for PDO drivers that support nextRowset()) greatly increases memory allocation to the script.

I've confirmed this problem with PHP 5.2.x and 6.

These results are from using PDO_ODBC with MS-SQL 2005. PDO_MYSQL and PDO_SQLITE would return 512 KiB no matter how many SELECT statements are run because they don't support nextRowset().

Reproduce code:
---------------
for ($i=0; $i<1000; $i++) {
  $result = $conn->query('SELECT * FROM foo');
  $result->closeCursor();
}
$mem = memory_get_peak_usage(true) / 1024;
echo $mem;

// Following run in a separate script to reset mem usage
for ($i=0; $i<100000; $i++) {
  $result = $conn->query('SELECT * FROM foo');
  $result->closeCursor();
}
$mem = memory_get_peak_usage(true) / 1024;
echo $mem;

Expected result:
----------------
512

512

Actual result:
--------------
1536

111872