PHP联系表单，如果选中CheckBox，则在电子邮件中为True / False

I have a file called send_form_email.php and I'm trying to get it to send an e-mail to me with trues/falses or some way of signifying which checkboxes are checked. I think that there's something wrong with the way I've tried to expand something I found on the internet - right now it sends the form okay, but just not any information about which items were checked.

These are the ones that are checkboxes on my web form:

$farmscale = $_POST['farmscale']; // not required
$nutrients = $_POST['nutrients']; // not required
$smallscale = $_POST['smallscale']; // not required
$plantdesign = $_POST['plantdesign']; // not required
$industrialscale = $_POST['industrialscale']; // not required
$engineering = $_POST['engineering']; // not required
$stgs = $_POST['stgs']; // not required
$compsupply = $_POST['compsupply']; // not required

But this is the main file, and I'm just not sure how to get a true/fales value to spit out in the email that it sends. Please be gentle... still kinda new to this stuff.

    <?php
    if(isset($_POST['email'])) {

        // EDIT THE 2 LINES BELOW AS REQUIRED
        $email_to = "michaelomchenry@gmail.com";
        $email_subject = "Inquiry from EnginuityEnergy.com";


        function died($error) {
            // your error code can go here
            echo "We are very sorry, but there were error(s) found with the form you submitted. ";
            echo "These errors appear below.<br /><br />";
            echo $error."<br /><br />";
            echo "Please go back and fix these errors.<br /><br />";
            die();
        }

        // validation expected data exists
        if(!isset($_POST['first_name']) ||
            !isset($_POST['last_name']) ||
            !isset($_POST['email']) ||
            !isset($_POST['telephone']) ||
            !isset($_POST['country']) ||
            !isset($_POST['comments'])) {
            died('We are sorry, but there appears to be a problem with the form you submitted.');       
        }

        $first_name = $_POST['first_name']; // required
        $last_name = $_POST['last_name']; // required
        $email_from = $_POST['email']; // required
        $telephone = $_POST['telephone']; // not required
        $country = $_POST['country']; // not required
        $comments = $_POST['comments']; // required
        $farmscale = $_POST['farmscale']; // not required
        $nutrients = $_POST['nutrients']; // not required
        $smallscale = $_POST['smallscale']; // not required
        $plantdesign = $_POST['plantdesign']; // not required
        $industrialscale = $_POST['industrialscale']; // not required
        $engineering = $_POST['engineering']; // not required
        $stgs = $_POST['stgs']; // not required
        $compsupply = $_POST['compsupply']; // not required

        $error_message = "";
        $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
      if(!preg_match($email_exp,$email_from)) {
        $error_message .= 'The Email Address you entered does not appear to be valid.<br />';
      }
        $string_exp = "/^[A-Za-z .'-]+$/";
      if(!preg_match($string_exp,$first_name)) {
        $error_message .= 'The First Name you entered does not appear to be valid.<br />';
      }
      if(!preg_match($string_exp,$last_name)) {
        $error_message .= 'The Last Name you entered does not appear to be valid.<br />';
      }
      if(strlen($comments) < 2) {
        $error_message .= 'The Comments you entered do not appear to be valid.<br />';
      }
      if(strlen($error_message) > 0) {
        died($error_message);
      }
        $email_message = "Form details below.

";

        function clean_string($string) {
          $bad = array("content-type","bcc:","to:","cc:","href");
          return str_replace($bad,"",$string);
        }

        $email_message .= "First Name: ".clean_string($first_name)."
";
        $email_message .= "Last Name: ".clean_string($last_name)."
";
        $email_message .= "Email: ".clean_string($email_from)."
";
        $email_message .= "Telephone: ".clean_string($telephone)."
";
        $email_message .= "Comments: ".clean_string($comments)."
";
        $email_message .= "Country: ".clean_string($country)."
";
        $email_message .= "Farm Scale: ".$farmscale."
";
        $email_message .= "Nutrients: ".$nutrients."
";
        $email_message .= "Small Scale: ".$smallscale."
";
        $email_message .= "Plant Design: ".$plantdesign."
";
        $email_message .= "Industrial Scale: ".$industrialscale."
";
        $email_message .= "Engineering: ".$engineering."
";
        $email_message .= "STGS: ".$stgs."
";
        $email_message .= "Component Supply: ".$compsupply."
";


    // create email headers
    $headers = 'From: '.$email_from."
".
    'Reply-To: '.$email_from."
" .
    'X-Mailer: PHP/' . phpversion();
    @mail($email_to, $email_subject, $email_message, $headers);  
    ?>

    <!-- include your own success html here -->

    <center>
        <h1>Thank you for contacting us.</h1>
        <br><br> 
        <p>We will be in touch with you very soon.</p>
        <p>In the meantime, head back to the <a href="http://www.enginuityenergy.com">Main Site</a> to learn more about Enginuity Energy's innovative Biomass gasification.<p>
    </center>


    <?php
    }
    ?>