Enviar evento iCalendar por email con mensaje

Aquí dejo una clase de PHP que hice  después de horas buscando en Google de  cómo enviar un evento ICalendar por email con mensaje y que funciona en Outlook:

<?php

class Ical {
    private $creation_date;
    private $start_date;
    private $end_date;
    private $subject;
    private $organizer_name;
    private $organizer_email;
    private $description;

    /**
     * Constructor - Sets ICalendar Preferences
     *
     * The constructor can be passed an array of config values
     */
    public function __construct($config = array())
    {
        $this->creation_date = $this->timestamp_to_cal(time());
        $this->start_date = $this->timestamp_to_cal(time());
        $this->end_date = $this->timestamp_to_cal(time()+3600);
        $this->subject = '';
        $this->organizer_name = '';
        $this->organizer_email = '';
        $this->description = '';
        if (count($config) > 0)
        {
            $this->initialize($config);
        }
    }

    //-._-._-._-._-._-._-._-._MAIN FUNCTIONS-._-._-._-._-._-._-._-._//

    // --------------------------------------------------------------------

    /**
     * Initialize preferences
     *
     * @access  public
     * @param   array
     * @return  void
     */
    public function initialize($config = array())
    {
        foreach ($config as $key => $val)
        {
            if (isset($this->$key))
            {
                $method = 'set_'.$key;

                if (method_exists($this, $method))
                {
                    $this->$method($val);
                }
                else
                {
                    $this->$key = $val;
                }
            }
        }

        return $this;
    }

    public function send_ical_event_email($email_from, $email_to, $subject, $html_message) {

        //Create unique identifier
        $cal_uid = date('Ymd').'T'.date('His')."-".rand()."@tudominio.com";

        //Create Mime Boundry
        //Sirve para separar los tipos de contenidos (iCalendar y el cuerpo del mensaje)
        $mime_boundary = "----lo_que_quieras----".md5(time());

        //Create Email Headers
        $headers = "From: Nombre <".$email_from.">\n";
        $headers .= "Reply-To: Nombre <".$email_from.">\n";

        $headers .= "MIME-Version: 1.0\n";
        $headers .= "Content-Type: multipart/alternative; boundary=\"$mime_boundary\"\n";
        $headers .= "Content-class: urn:content-classes:calendarmessage\n";

        //Create Email Body (HTML)
        $message = "--$mime_boundary\n";
        $message .= "Content-Type: text/html; charset=UTF-8\n";
        $message .= "Content-Transfer-Encoding: 8bit\n\n";

        $message .= $html_message;
        $message .= "\n";
        $message .= "--$mime_boundary\n";

        //Create ICAL Content (Google rfc 2445 for details and examples of usage)
        $ical = 'BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 11.0 MIMEDIR//EN
VERSION:2.0
METHOD:REQUEST
BEGIN:VEVENT
ORGANIZER:MAILTO:'.$email_from.'
DTSTART:'.$this->start_date.'
DTEND:'.$this->end_date.'
TRANSP:OPAQUE
SEQUENCE:0
UID:'.$cal_uid.'
DTSTAMP:'.$this->creation_date.'
DESCRIPTION:'.$this->description.'
SUMMARY:'.$this->subject.'
PRIORITY:5
CLASS:PUBLIC
END:VEVENT
END:VCALENDAR';

        $message .= 'Content-Type: text/calendar;name="meeting.ics";method=REQUEST;charset=utf-8\n';
        $message .= "Content-Transfer-Encoding: 8bit\n\n";
        $message .= $ical;            

        //SEND MAIL
        $mail_sent = @mail($email_to, $subject, $message, $headers );

        if($mail_sent)     {
            return true;
        } else {
            return false;
        }   

    }

    //-._-._-._-._-._-._-._-._HELPER FUNCTIONS-._-._-._-._-._-._-._-._//
    public function x_date_to_cal($date){
        if(is_numeric($date)){
            return $this->timestamp_to_cal($date);
        }else{
            return $this->date_to_cal($date);
        }
    }

    public function date_to_cal($date_str) {
        return $this->timestamp_to_cal(strtotime($date_str));
    }

    public function timestamp_to_cal($timestamp){
        return gmdate('Ymd\THis\Z', $timestamp);
    }

    // Escapes a string of characters
    public function escape_string($string) {
        $string = preg_replace('/([\,;])/','\\\$1', $string);
        return html_entity_decode($string, ENT_QUOTES, 'UTF-8');
    }

    //-._-._-._-._-._-._-._-._SETTERS-._-._-._-._-._-._-._-._//
    public function set_creation_date($date){
        $this->creation_date = $this->x_date_to_cal($date);
        return $this;
    }

    public function set_start_date($date){
        $this->start_date = $this->x_date_to_cal($date);
        return $this;
    }

    public function set_end_date($date){
        $this->end_date = $this->x_date_to_cal($date);
        return $this;
    }

    public function set_subject($subject){
        $this->subject = $this->escape_string($subject);
        return $this;
    }

    public function set_organizer_name($name){
        $this->organizer_name = $this->escape_string($name);
        return $this;
    }

    public function set_organizer_email($email){
        $this->organizer_email = $this->escape_string($email);
        return $this;
    }

    public function set_description($description){
        $description = strip_tags($description);
        $this->description = $this->escape_string($description);
        return $this;
    }
}

Para usarla basta con hacer lo siguiente:

 

<?php

$icalendar = new class Ical(array(
                'start_date' => '12-10-2014 17:00', //También sirven timestamp, o fechas en cualquier otro formato válido en PHP
                'end_date' => '12-10-2014 19:00',
                'subject' => 'Asunto',
                'organizer_name' => 'Nombre del organizador del evento',
                'organizer_email' => 'organizador@dominio.com',
                'description' => 'Descripción del evento'
                ));

if($icalendar->send_ical_event_email('email_desde@dominio.com', 'email_destino@dominio.com', 'Asunto del email', 'Mensaje del email')) {
    echo 'Email envíado';
}else{
    echo 'Error al enviar email';
}

Más información sobre ICalendar:
http://en.wikipedia.org/wiki/ICalendar