How to build a format for email body with HTML sent with PHP

Trying to understand how to customize an email sent with PHP. I believe its suppose to be done with basic <html> and tables. What am I doing wrong in the $messagearea?

As of now I get this in my message area:

<html><body><h1>Hello, World!</h1></body></html>

Here is my code:

<?php
  session_start();
  require_once('payflow.php');

  $tranid  = $_GET['PNREF'];
  $email   = $_GET['EMAIL'];
  $billtofirstname = $_GET['BILLTOFIRSTNAME'];
  $billtolastname = $_GET['BILLTOLASTNAME'];
  $billtoname = $_GET['BILLTONAME'];
  $billtostreet = $_GET['BILLTOSTREET'];
  $billtostreet2 = $_GET['BILLTOSTREET2'];
  $billtocity = $_GET['BILLTOCITY'];
  $billtostate = $_GET['BILLTOSTATE'];
  $billtozip = $_GET['BILLTOZIP'];

  $to = $email;
  $subject = "ORDER #$tranid/n";
  $headers = "From: $email/n";

  $message = '<html><body>';
  $message .= '<h1>Hello, World!</h1>';
  $message .= '</body></html>';

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

?>

Email works well, $_GET data is fine. Just confussed on how to get the message area to read <html>

As per php document,

<?php
// Multiple recipients
$to = '[email protected], [email protected]'; // note the comma

// Subject
$subject = 'Birthday Reminders for August';

// Message
$message = '
<html>
<head>
  <title>Birthday Reminders for August</title>
</head>
<body>
  <p>Here are the birthdays upcoming in August!</p>
  <table>
    <tr>
      <th>Person</th><th>Day</th><th>Month</th><th>Year</th>
    </tr>
    <tr>
      <td>Johny</td><td>10th</td><td>August</td><td>1970</td>
    </tr>
    <tr>
      <td>Sally</td><td>17th</td><td>August</td><td>1973</td>
    </tr>
  </table>
</body>
</html>
';

// To send HTML mail, the Content-type header must be set
$headers[] = 'MIME-Version: 1.0';
$headers[] = 'Content-type: text/html; charset=iso-8859-1';

// Additional headers
$headers[] = 'To: Mary <[email protected]>, Kelly <[email protected]>';
$headers[] = 'From: Birthday Reminder <[email protected]>';
$headers[] = 'Cc: [email protected]';
$headers[] = 'Bcc: [email protected]';

// Mail it
mail($to, $subject, $message, implode("/r/n", $headers));
?>