Getting value of SQL Server Output parameter using PDO & ODBC

I've been learning PHP by adding some functionality to one of my company's web pages to validate user input of SSN before inserting a new record into the database. That part is working fine. The database insert involves calling a SQL Server stored procedure that has an output parameter that returns the GUID of the new record for use in a subsequent process. The database insert part works but when I echo the value of the returned parameter it's an empty string.

Here are pertinent details: SQL Server 2008 running on a separate server WAMP Server 2.2 running on a Windows 64 PC PHP 5.3.1 Apache 2.2.22

Here is my PHP code:

$conn = new PDO( "odbc:Driver={SQL Server};Server=xxx.xxx.xx.xx;Database=xxxxx;charset=UTF-8'", "uid", "pwd");

if(!$conn) {
    die( "<br />Error connecting to SQL Server");
}

$conn->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );

$resource_id = "";

$stmt = $conn->prepare("{CALL xxxx.dbo.app_sp_insert_rq(?, ?, ?, ?, ?, ?, ?, ?)}");

$stmt->bindParam(1, $survey_text, PDO::PARAM_STR);
$stmt->bindParam(2, $event_date, PDO::PARAM_STR);
$stmt->bindParam(3, $type_id, PDO::PARAM_STR);
$stmt->bindParam(4, $description, PDO::PARAM_STR);
$stmt->bindParam(5, $folder_id, PDO::PARAM_STR);
$stmt->bindParam(6, $has_details, PDO::PARAM_STR);
$stmt->bindParam(7, $uid, PDO::PARAM_STR);
$stmt->bindParam(8, $resource_id, PDO::PARAM_STR|PDO::PARAM_INPUT_OUTPUT, 100); 

$stmt->execute();

echo $resource_id;

Here is the stored procedure definition:

CREATE PROCEDURE [dbo].[app_sp_insert_rq]
    @survey_text nvarchar(max),
    @event_date nvarchar(20),
    @type_id nvarchar(100),
    @description nvarchar(100),
    @folder_id nvarchar(100),
    @has_details nchar(1),
    @uid nvarchar(100),
    @resource_id nvarchar(100) output

AS

select @resource_id = NEWID()

insert into
    xxxxx.dbo.app_events (
        resource_id,
        event_date, 
        [type_id], 
        [description], 
        folder_id, 
        has_details, 
        [uid], 
        survey_text
    )
    values (
        @resource_id,
        @event_date, 
        @type_id, 
        @description, 
        @folder_id, 
        @has_details, 
        @uid, 
        @survey_text
    )

Any help will be much appreciated. I only got this far via multiple searches but this is one roadblock I haven't been able to figure a way around.