我试图添加一个reCAPTCHA到我的网站，但不断得到incorrect-captcha-sol错误时，我提交的答案。
谁能告诉我，我做以下事情是否正确？
我有一个通用的index.php，其中包含contact.php。在contact.php中，我插入了以下代码：

require_once('recaptchalib.php');
$publickey = "XXXX";
$privatekey = "XXXX";

//the response from reCAPTCHA
$resp = null;
//the error code from reCAPTCHA, if any
$error = null;

if ($_POST['submit']) {
    $message = $_POST['message_txt'];
    $name = $_POST['name_txt'];
    $email = $_POST['email_txt'];

    $emailBody = $message;
    $to = 'xx';
    $from = $name.' <'.$email.'>';
    $subject = 'XX Website Enquiry';
    $headers = 'From: '.$from;  

    $resp = recaptcha_check_answer($privatekey, $_SERVER["REMOTE_ADDR"], $_POST["recaptcha_challenge_field"], $_POST["recaptcha_response_field"]);

    if ($resp->is_valid) {
        echo 'captcha correct';
        if (mail($to,$subject,$emailBody,$headers)) {
            //echo 'mail sent';
            $confirmation = 'sent';
        }
        else {
            //echo 'mail not sent';
            $confirmation = 'error';
        }
    } else {
        # set the error code so that we can display it. You could also use
        # die ("reCAPTCHA failed"), but using the error message is
        # more user friendly

        $error = $resp->error;

        echo $error;
    }
}

在我的html中，我插入了如下的验证码：

<form name="contactForm" method="post" action="index.php?id=contact&action=submit#contact">
    <tr><td>Name</td><td><div align="right">
        <input type="text" name="name_txt" class="input">
        </div></td></tr>
    <tr><td>Email</td><td><div align="right">
        <input type="text" name="email_txt" class="input">
    </div></td></tr>
    <tr><td height="10"></td></tr>
    <tr><td colspan="2">Message</td></tr>
    <tr><td colspan="2"><textarea name="message_txt" class="textarea" style="width:200px; height:100px"></textarea></td></tr>
    <tr><td colspan="2"><?php echo recaptcha_get_html($publickey, $error); ?></td></tr>
    <tr><td colspan="2" style="padding-top:10px;"><input type="image" src="images/header_06.gif" name="submit" value="submit"></td></tr>
</form>

我看不出我做错了什么，但我会感激任何帮助。