将PHP Authenticated邮件更改为HTML Body

i have this PHP Code/function to send emails.

what code do i need to add to make the emails that get sent HTML rather than plain Text?

function sendemail($email_to,$email_from,$email_subject,$email_body,$email_replyto)
    {
        require_once "/usr/local/lib/php/Mail.php";

        $from = $email_from;
        $to = $email_to;
        $subject = $email_subject;
        $body = $email_body;

        $host = "mail.integradigital.co.uk";
        $username = "sending@integradigital.co.uk";
        $password = "*********";

        $headers = array ('From' => $from,
          'To' => $to,
          'Subject' => $subject);
        $smtp = Mail::factory('smtp',
          array ('host' => $host,
         'auth' => true,
         'username' => $username,
         'password' => $password));

        $mail = $smtp->send($to, $headers, $body);

        if (PEAR::isError($mail)) {
          echo("<p>" . $mail->getMessage() . "</p>");
         } else {
          echo("<p>Message successfully sent!</p>");
         }
    }

add "Content-Type: text/html; charset=ISO-8859-1 "; to your headers.

<?php
// multiple recipients
$to  = 'aidan@example.com' . ', '; // note the comma
$to .= 'wez@example.com';

// 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>Joe</td><td>3rd</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 <mary@example.com>, Kelly <kelly@example.com>' . "
";
$headers .= 'From: Birthday Reminder <birthday@example.com>' . "
";
$headers .= 'Cc: birthdayarchive@example.com' . "
";
$headers .= 'Bcc: birthdaycheck@example.com' . "
";

// Mail it
mail($to, $subject, $message, $headers);
?>

Refer the link http://php.net/manual/en/function.mail.php

You'd be better off using the Mail_Mime package which will allow you to send both HTML and plain text e-mails easily:-

$mime = new Mail_mime();
$mime->setTXTBody($text);
$mime->setHTMLBody($html);

Here's a good example from the documentation