PHP Sql Server Output Parameter

I'm struggling with an output parameter in PHP and SQL Server 2008. I keep getting the error:

Notice: Undefined variable: UserID in C:\inetpub\wwwroot\PersonalWebsitePHP\Register.php on line 67 Error in executing statementArray ( [0] => Array ( [0] => IMSSP [SQLSTATE] => IMSSP [1] => -7 [code] => -7 [2] => An invalid PHP type was specified as an output parameter. DateTime objects, NULL values, and streams cannot be specified as output parameters. [message] => An invalid PHP type was specified as an output parameter. DateTime objects, NULL values, and streams cannot be specified as output parameters. ) )

My code:

$con = sqlsrv_connect(".\SQLExpress", $connectionInfo);
                            if ($con === false) {
                                echo "Could not connect \n";
                                die(print_r(sqlsrv_errors(), true));
                            }

                            $params = array(
                                            array($_POST["FirstName"], SQLSRV_PARAM_IN),
                                            array($_POST["LastName"], SQLSRV_PARAM_IN),
                                            array($_POST["Email"], SQLSRV_PARAM_IN),
                                            array($_POST["Username"], SQLSRV_PARAM_IN),
                                            array(md5($_POST["Password"]), SQLSRV_PARAM_IN),
                                            array(date("Y-m-d H:i:s"), SQLSRV_PARAM_IN),
                                            array($_SERVER["REMOTE_ADDR"], SQLSRV_PARAM_IN),
                                            array("Member", SQLSRV_PARAM_IN),
                                            array("No", SQLSRV_PARAM_IN),
                                            array($UserID, SQLSRV_PARAM_OUT)
                                           );
                            $tsql_callSP = "{call InsertUser(?,?,?,?,?,?,?,?,?,?)}";
                            $stmt3 = sqlsrv_query($con, $tsql_callSP, $params);
                            if ($stmt3 === false) {
                                echo "Error in executing statement";
                                die(print_r(sqlsrv_errors(), true));
                            }
                            sqlsrv_next_result($stmt3);
                            echo "Hi " . $_POST["FirstName"] . ", your User ID is " . $UserID . ".";

                        }
                    } else {
                        die ("The reCAPTCHA wasn't entered correctly.  Go back and try it again.");

                    }

I followed the tutorials exactly. When I declare $UserID before the array, it just prints out the value declared in the variable rather than the output parameter. The input parameters work fine since I can see them in the database. Hope someone can help with this one.

Thank you

Douglas