PDO问题，无错误消息

我有以下代码片段：

error_reporting(E_ALL);
$hostname = "localhost";
$username = "user";
$password = "password";
try {
    $db = new PDO("mysql:host=$hostname;dbname=DBNAME", $username, $password);
    echo "connected...<br/>'n";flush();
    $sql = "SELECT COLA, COLB FROM TABLEA LIMIT 10";
    echo "query:$sql--<br/>'n";flush();
    $stmt = $dbh->query($sql);
    echo "statement:<pre>";print_r($stmt);echo "</pre>'nfetching...<br/>'n";flush();
    $result = $stmt->fetch(PDO::FETCH_ASSOC);
    if ($result === true) {
        echo "result is true<br/>'n";flush();
    }
    else if ($result === false) {
        echo "result is false<br/>'n";flush();
    }
    else {
        echo "what?<br/>'n";flush();
    }
    foreach ($result as $key=>$val) {
        echo "key:$key - val:$val--<br/>'n";flush();
    }
    echo "done";
    $dbh = null;
}
catch (Exception $e) {
    die ($e->getMessage());
}

正如你所看到的，我有一堆调试echo语句，但我看到的只有：

connected...
query:SELECT COLA, COLB FROM TABLEA LIMIT 10

我本来希望看到我的语句对象以及我的结果被显示，我是不是遗漏了什么？