PHP 5/MySQL 5 issues with Output parameters from PDO

Hey everyone...
I'm having an issue with a seemingly simple piece of PHP/MySQL

I have a stored procedure in MySQL 5 as such:

SQL:
--------------
DELIMITER $$;
DROP PROCEDURE IF EXISTS `test`.`sp_returns_string`$$
CREATE PROCEDURE `test`.`sp_returns_string`(OUT vOutput varchar(32))
BEGIN
SET vOutput = 'It Worked';
END$$
DELIMITER ;$$
--------------

And a piece of PHP (v5.1) that uses PDO to call the Stored Proc as such:

PHP:
--------------
<?php

//First - check/load PDO!
if (!extension_loaded('pdo_mysql')) {
// If not loaded we could try loading it manually
$prefix = (PHP_SHLIB_SUFFIX == 'dll') ? 'php_' : '';
if (!@dl($prefix . 'pdo_mysql.' . PHP_SHLIB_SUFFIX)) {
die('pdo_mysql unavailable');
}
}

$DB_host = "localhost"; // the hostname of the database server
$DB_user = "root"; // YOUR username to connect with
$DB_pass = "password"; // YOUR user's password
$DB_dbName = "test"; // the name of the database to connect to
$DB_dbType = "mysqli"; // the type of database server.

$DB_Con = "mysql:host=$DB_host;dbname=$DB_dbName";
$dbOptions = array();

//Create a DB connection
$db = new PDO($DB_Con, $DB_user, $DB_pass, $dbOptions);

$return_value = '';
$stmt = $db->prepare("CALL sp_returns_string(@?)");
$stmt->bindParam(1, $return_value, PDO::PARAM_STR, 4000);
$stmt->execute();
print_r($stmt);
print "<br/>Returned: $return_value<br/><br/>\r\n";
?>
--------------

This should print something like this:
--------------
PDOStatement Object ( [queryString] => CALL sp_returns_string(@?) )
Returned: It Worked!
--------------

But unfortunately it NEVER returns anything.
The Stored Proc works fine and returns the string ok, but only when I'm
calling it from an SQL console.

The PHP PDO documentation says to use:
--------------
$return_value = '';
$stmt = $db->prepare("CALL sp_returns_string(?)");
$stmt->bindParam(1, $return_value, PDO::PARAM_STR, 4000);
--------------
Note the missing "@".
But when I do this, it complains that the parameter isn't a variable and
the call dies.

(see: http://www.php.net/pdo/ for the examples)

Any help would be great!

Thanks.
James.