PDO链接MySQL问题

[Soulic]

在创建PDO对象的时候提示: SQLSTATE[HY000] [1045] Access denied for user 'root'@'localhost' (using password: YES)
代码如下:

$user = 'root';
$password = '123456';
$dbName = 'test';

try {
    $dbh = new PDO('mysql:host=127.0.0.1;dbname=' . $dbName, $user, $password);
} catch (PDOException $e) {
    echo 'Connection failed: ' . $e->getMessage();
}

使用mysqli连接时却是可以的

$user = 'root';
$password = '123456';
$dbName = 'test';

$mysqli = new mysqli('localhost', $user, $password, $dbName);
if ($mysqli->connect_error) {
    die('Connect Error (' . $mysqli->connect_errno . ') '
        . $mysqli->connect_error);
}

echo 'Success... ' . $mysqli->host_info . "\n";

$mysqli->close();

有谁知道是什么原因吗？谢谢