function send_message($message, $chat_room_id, $sender_id, $recipient_id = -1, $file = null)
 {
     $db =& db_factory::instance();
     $time = time();
     $message = htmlspecialchars($message);
     $file_id = chat_system::add_file($file, $chat_room_id, $sender_id);
     $sql = "INSERT INTO chat_message \r\n\t\t\t\t\t\t(message, chat_room_id, time, sender_id, recipient_id, file_id)\r\n\t\t\t\t\t\tvalues\r\n\t\t\t\t\t\t('{$message}', '{$chat_room_id}', {$time}, '{$sender_id}', \r\n\t\t\t\t\t\t\t'{$recipient_id}', '{$file_id}')";
     $db->sql_exec($sql);
 }