MS SQL/FreeTDS failing with a simple SQL SET

Hi,

I have identified a problem with a MS SQL query I am running with the 
freetds driver, PDO and PHP.

Basically, if I send the query:

DECLARE @A INT
SET @A = 1
SELECT 2 AS b

It fails. But if I remove the SET it works. The code looks like this:

<?php

$old_dbhost="somehost";
$old_dbuser="auser";
$old_dbpass="topsecret";
$old_database="database";

try {
$dbh = new PDO("dblib:host=$old_dbhost;dbname=$old_database", 
$old_dbuser, $old_dbpass);
} catch (PDOException $e) {
echo "Error!: " . $e->getMessage() . "<br/>";
exit();
}

// Uncomment the SET below, and the query will work

$sql="DECLARE @A INT
-- SET @A = NULL
SELECT 2 AS B
";

try {
$sth=$dbh->prepare("$sql");
} catch (PDOException $e) {
echo "prepare Error!: " . $e->getMessage() . "<br/>";
}

try {
$sth->execute();
} catch (PDOException $e) {
print "Error!: " . $e->getMessage() . "<br/>";
die();
}

try {
$records=$sth->fetchAll(PDO::FETCH_ASSOC);
} catch (PDOException $e) {
print "Error!: " . $e->getMessage() . "<br/>";
die();
}
unset($sth);

print_r($records);

?>

When it fails (with the SET uncommented) the freetds log contains:

11:33:45.967347 3683 (dblib.c:4565):dbsqlok() not done, calling 
tds_process_tokens()

But with the SET in place, the log contains:

11:32:18.751088 3671 (dblib.c:4556):dbsqlok() exits on result token 0xa0

I think this would suggest a syntax error with the SQL, but I have run 
it directly on the server, and it works fine.

Versions:
FreeTDS 0.82
SQL Server: 2008 R2
PHP: 5.3.3

Any ideas anyone?

Cheers,

Chris.