This also happens with "normal" queries executed via $pdo->query('..'), not only with prepared statements.

We at ownCloud are facing the same issue.

Looks like the php 5.3 code base for pdo_dblib is fetch all rows within pdo_dblib_stmt_execute() - see https://github.com/php/php-src/blob/php-5.3.28/ext/pdo_dblib/dblib_stmt.c#L145

Might that be related to this comment in the freedts docs: http://www.freetds.org/userguide/samplecode.htm (search for 'Fetch All Rows!')

Any further guidance is welcome - we are happy to 'donate' developer time to this issue - if this helps ;-)

Same thing happening on Ubuntu 14.04, PHP 5.5.9-1ubuntu4.2

Here's a test script that doesn't require a table, if that helps:

Test script:
---------------

$dsn      = 'dblib:dbname=DB_NAME;host=DB_HOST';

$db = new \PDO($dsn, 'USER', 'PASSWORD');

$statement = $db->prepare('SELECT 1 UNION SELECT 2 UNION SELECT 3');
$statement->execute();

while($row = $statement->fetch())
{    
    $statement2 = $db->prepare('SELECT 1');
    $statement2->execute();
    $statement2->fetch();

    // Loop will exit
}

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This is the behavior of MSSQL (TDS), DBLIB and FreeTDS. One statement per connection rule. If you initiate another statement, the previous statement is cancelled. 

The previous versions buffered the entire result set in memory leading to OOM errors on large results sets. 

The previous behavior can be replicated using fetchAll() and a loop if desired. Another workaround is to open 2 connection objects, one per statement.