关于pdo的一个疑问,该如何解决

关于pdo的一个疑问

PHP code

if (isset($_POST['loginName']) && isset($_POST['pwd']) && $_POST['action'] ==    'submit') {    $loginName = $_POST['loginName'];    $pwd = $_POST['pwd'];        echo $loginName.' '.$pwd;    $sql = "SELECT COUNT(*) FROM tb_esq WHERE login_name = :loginName AND pwd = :pwd";    require_once ('conn.php');    $sth = $db->prepare($sql);    $sth->bindParam(':loginName', $loginName, PDO::PARAM_STR, 25);    $sth->bindParam(':pwd', $pwd, PDO::PARAM_STR, 25);    $sth->execute();    $rowCount = $sth->fetchColumn();    var_dump($rowCount);//一直是0    $db = NULL;    if ($rowCount == 1) {        echo "<script>window.location=\"index.html\";</script>";    } else {        alert('用户名或密码错误!');        echo "<script>window.location=\"login.php\";</script>";    }}

PHP code

$sql = "SELECT COUNT(*) FROM tb_esq WHERE login_name = '$loginName' AND pwd = '$pwd'";$result = $db->query($sql);$row = $result->fetch(PDO::FETCH_ASSOC);print_r($row);