Amazon RDS PHP MSSQL

Я пытаюсь открыть соединение с Amazon RDS MS-SQL от хостинга с MSSQL библиотека со следующим кодом:

<?php
$dbhost  = "xxxxxxx.rds.amazonaws.com";
$dbport  = 1433;
$dbname  = "xxxxx";
$dbuser  = "xxxxxx";
$dbpass  = "xxxxxxx";
try {
$dbh = new PDO ("dblib:host=$dbhost:$dbport;dbname=$dbname","$dbuser","$dbpass");
} catch (PDOException $e) {
echo "Failed to get DB handle: " . $e->getMessage() . "\n";
exit;
}

и я получаю эту ошибку:

Failed to get DB handle: SQLSTATE[HY000] Unable to connect: Adaptive Server is unavailable or does not exist (severity 9)