请大神看看，这连接数据库的代码有问题么

<?php
$dbms = 'mssql';
$host = 'localhost';
$dbName = 'A8';
$user = 'sa';
$pass = '123456';
$dsn = "$dbms:host=$host;dbname=$dbName";
try {
$pdo = new PDO($dsn, $user, $pass);
echo "连接成功！";
} catch (PDOException $e) {
die("error!:" . $e->getMessage() . "<br/>");
}
?>
一直出现error!:could not find driver