$params = explode('.', $_SERVER['HTTP_HOST']);

if($params[0] == 'www') {
   // www exists
}

<?php

if (substr($_SERVER['HTTP_HOST'], 0, 3) == 'www') {
    header('HTTP/1.1 301 Moved Permanently');
    header('Location: http://www.you-site.ru');
    die;
}