单选按钮:PHP只传递"Other"价值

我借用了几个版本的PHP(从这里和其他网站)来传递无线电字段集中的选定金额到电子商务网关，但无论我尝试了什么，它只会传递输入到我的"其他"字段(这是一个简单的文本框)的金额。选中任何一个单选按钮后，我都会得到一个错误，表示字段缺失或格式错误。这是我第一次使用PHP，所以任何在初级水平上提供的帮助都是非常感谢的!虽然这个问题似乎与if else if语句有关，但我还是包含了整个PHP，因为我并不真正了解PHP是如何工作的。谢谢。

    <?php 
$x_login = "HCO-ST.-T-902"; //  Hosted Payment Page ID. 
$transaction_key = "6~~xpvR~xMJXN_RkCc99"; // 
if(isset($_POST['amount'])) {
    if($_POST['amount'] == '5') {
        $x_amount = "5.00";
    } elseif($_POST['amount'] == '10') {
        $x_amount = "10.00";
    } elseif($_POST['amount'] == '25') {
        $x_amount = "25.00";
    } elseif($_POST['amount'] == '50') {
        $x_amount = "50.00";
    } elseif($_POST['amount'] == '100') {
        $x_amount = "100.00";
    } elseif($_POST['amount'] == '200') {
        $x_amount = "200.00";
    } elseif($_POST['amount'] == 'other') {
        $x_amount = $_POST['amount']; 
    }
}
$x_invoice_num = $_POST['invoice'];
$x_first_name = $_POST['x_first_name'];
$x_email = $_POST['x_email'];
$CardHoldersName= $_POST['CardHoldersName'];
$x_currency_code = "USD"; // Needs to agree with the currency of the payment page
srand(time()); // initialize random generator for x_fp_sequence
$x_fp_sequence = rand(1000, 100000) + 123456;
$x_fp_timestamp = time(); // needs to be in UTC. Make sure webserver produces UTC
// The values that contribute to x_fp_hash 
$hmac_data = $x_login . "^" . $x_fp_sequence . "^" . $x_fp_timestamp . "^" . $x_amount . "^" . $x_currency_code;
$x_fp_hash = hash_hmac('MD5', $hmac_data, $transaction_key);
echo ('<input type="hidden" name="x_login" value="' . $x_login . '">' );
echo ('<input type="hidden" name="x_amount" value="' . $x_amount . '">' );
echo ('<input type="hidden" name="x_fp_sequence" value="' . $x_fp_sequence . '">' );
echo ('<input type="hidden" name="x_fp_timestamp" value="' . $x_fp_timestamp . '">' );
echo ('<input type="hidden" name="x_fp_hash" value="' . $x_fp_hash . '" size="50">' );
echo ('<input type="hidden" name="x_currency_code" value="' . $x_currency_code . '">');
echo ('<input type="hidden" name="x_invoice_num" value="' . $x_invoice_num . '">');
echo ('<input type="hidden" name="x_first_name" value="' . $x_first_name . '">');
echo ('<input type="hidden" name="x_email" value="' . $x_email . '">');
?>
<input type="hidden" name="x_show_form" value="PAYMENT_FORM"/>
</form>

<fieldset name="amount">
<legend><h3>Amount**</h3></legend>
<hr class="hr">
<p style="margin:0">
<input name="amount" type="radio" value="5" checked="checked"> $5.00
<input name="amount" type="radio" value="10"> $10.00
<input name="amount" type="radio" value="25"> $25.00<br>
<input name="amount" type="radio" value="50"> $50.00
<input name="amount" type="radio" value="100"> $100.00
<input name="amount" type="radio" value="200"> $200.00
</p>
<input name="amount" type="radio" value="other"><span style="text-decoration: underline">Other Amount:  </span>
<input name="amount" type="text" style="width:100%">
</fieldset>