如何在 PHP 中使用 SMTP 发送 HTML 电子邮件

I've been able to send an email using SMTP in PHP, but when I try to change the Content Type to HTML, the email doesn't get delivered. This is the code I'm trying to use:

    require_once "Mail.php";

    $from = "FPM <forms@fpmofames.com>";
    $from_name = "FPM";

    $host = "localhost";
    $username = "username";
    $password = "password";

    $subject = "Subject";
    $message = "Message";

    $to = "<example@example.com>";
    $headers = array ('From' => $from,
        'To' => $to,
        'Subject' => $subject,
        'MIME-Version' => '1.0',
        'Content-Type' => "text/html; charset=ISO-8859-1"
    );

    $smtp = Mail::factory('smtp',
        array ('host' => $host,
            'auth' => true,
            'username' => $username,
            'password' => $password));

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

如果我从标头中取出Content-Type"参数，它会很好地发送消息.我不知道为什么添加它会导致问题.