PHP联系表单不从下拉菜单</select>中的<select>选项中检索结果

Question

需要帮助弄清楚为什么PHP没有从下拉列表中检索数据。发送值为“undefined”的消息 --- HTML ---
<select name="amount" id="amount" title="Please choose dollar amount.">
<option value="">- Choose dollar amount from a list -</option>
<option value="25">$25</option>
<option value="50">$50</option>
<option value="75">$75</option>
</select>

--- --- PHP
//vars
$amount = $_POST['amount'];

//data
$msg .= "AMOUNT: " .$_POST['amount'] ."<br>\n";

//send for each mail
foreach($to as $mail){
mail($mail, $subject, $msg, $headers);