Флажок проверки PHP

Я обыскал весь Интернет, пытаясь найти решение того, чего мне здесь не хватает (или что я делаю неправильно). Моя форма не проверяется, даже если установлен флажок. Все остальное работает нормально.

Это тот флажок, я не могу нормально работать.

Я пробовал много разных идей, но это не подтверждается даже при проверке «условий» (например, в этом примере ниже).

Вот мой HTML:

<div class = "form-check">
      <input type = "checkbox" class = "form-check-input" name = "terms" id = "terms"  />
     <label class = "form-check-label" for = "terms"><p>I agree to terms of service.</p></label>
</div>

Вот вся моя проверка PHP (обновленная комментариями / ответами ниже):

<?php

if (!$_POST) exit;

// Email address verification.
function isEmail($email) { // lots of email validation stuff in here // }

if (!defined("PHP_EOL")) define("PHP_EOL", "\r\n");

$name     = $_POST['name'];
$email    = $_POST['email'];
$website   = $_POST['website'];
$subject  = $_POST['subject'];
$comments = $_POST['comments'];
$terms = $_POST['terms'];


//set an error counter to trigger the `exit`
$error_counter = 0;
if (trim($name) == '') {
echo '<div class = "error_message">Attention! You must enter your name</div>';
$error_counter++;
} 
if (trim($email) == '') {
echo '<div class = "error_message">Attention! Please enter a valid email 
address.</div>';
$error_counter++;
}
if (!isEmail($email)) {
echo '<div class = "error_message">Attention! You have entered an invalid e- 
mail address, try again.</div>';
$error_counter++;
} 
if (trim($subject) == '') {
echo '<div class = "error_message">Attention! Please enter a subject.</div>';
$error_counter++;
}
if (trim($comments) == '') {
echo '<div class = "error_message">Attention! Please enter your message. 
</div>';
$error_counter++;
}
if (empty($terms)) {
echo '<div class = "error_message">Attention! Please agree to our terms of 
service.</div>';
$error_counter++;
}
//if `$error_counter > 0 > 0` it will trigger the `exit()` to stop the script and display the errors.
if ($error_counter > 0){
exit();
}

if (get_magic_quotes_gpc()) {
$comments = stripslashes($comments);
}


// Configuration option.
// Enter the email address that you want to emails to be sent to.
// Example $address = "[email protected]";

$address = "[email protected]";


// Configuration option.
// i.e. The standard subject will appear as, "You've been contacted by John Doe."

// Example, $e_subject = '$name . ' has contacted you via Your Website.';

$e_subject = 'You have been contacted by ' . $name . '.';


// Configuration option.
// You can change this if you feel that you need to.
// Developers, you may wish to add more fields to the form, in which case you must be sure to add them here.

$e_body = "You have been contacted by $name with regards to $subject, their additional message is as follows." . PHP_EOL . PHP_EOL;
$e_content = "\"$comments\"" . PHP_EOL . PHP_EOL;
$e_reply = "You can contact $name via email, $email ";

$msg = wordwrap( $e_body . $e_content . $e_reply, 70 );

$headers = "From: $email" . PHP_EOL;
$headers .= "Reply-To: $email" . PHP_EOL;
$headers .= "MIME-Version: 1.0" . PHP_EOL;
$headers .= "Content-type: text/plain; charset=utf-8" . PHP_EOL;
$headers .= "Content-Transfer-Encoding: quoted-printable" . PHP_EOL;

if (mail($address, $e_subject, $msg, $headers)) {

// Email has sent successfully, echo a success page.

echo "<fieldset>";
echo "<div id='success_page'>";
echo "<h1>Email Sent Successfully.</h1>";
echo "<p>Thank you <strong>$name</strong>, your message has been submitted to us.</p>";
echo "</div>";
echo "</fieldset>";

} else {

echo 'ERROR!';

}

?>