邮件发送但不提交？

I am a PHP novice here. I have a send mail script that looks like this:

$to      = 'example@email.com';
$from    = 'noreply@email.com';
$subject = 'Test Submission';
$message = 'This is just another test.';
$headers = 'From: example@email.com' . "
" .
// 'Reply-To: example@email.com' . "
" .
'X-Mailer: PHP/' . phpversion();

mail($to, $subject, $message, $headers);

This script works fine and sends an e-mail when the page is loaded. The problem is that when I alter the script just enough so that the form is not submitted until the Submit button is clicked then all of a sudden nothing works anymore. Here is the altered code I have been trying to use (and which seems to go right along with what the PHP site suggests):

if(isset($_POST['submit'])){
    $to      = 'example@email.com';
    $from    = 'noreply@email.com';
    $subject = 'Test Submission';
    $message = 'This is just another test.';
    $headers = 'From: example@email.com' . "
" .
    // 'Reply-To: example@email.com' . "
" .
    'X-Mailer: PHP/' . phpversion();

    mail($to, $subject, $message, $headers);
    }

Make sure your submit button on your form has a NAME attribute. The value of that NAME attribute is what gets sent to the server, so:

<input type="submit" name="btnSubmit" value="Go!">

...would result in this variable and value:

$_POST["btnSubmit"] = "Go!";

...and you would check it like this:

if(isset($_POST["btnSubmit"]))
{
  ...
}