php强制用户转向www域名

有时候网站的www域名和非www域名都能访问网站，但是这样不利于搜索引擎的收录，会分散网页的权重，所以希望用户访问非www的域名时通过301永久重定向到www域名，例如用户访问75271.com会直接转向www.75271.com，本php代码考虑了无法通过head重定向的情况，会在页面上输出链接，让用户点击。

// Install info.:
// Copy and paste these lines into your default index.php or
// the file that get's called if a visitor comes on your 
// website...
 
// read the host from the server environment
$host = $_SERVER["HTTP_HOST"];
 
// fix host name - we never now... ;-)
$host = strtolower($host);
$host = trim($host);
 
// This is important: 
// Webbrowsers like Firefox are doing their request without
// the port number like "www.jonasjohn.de" but some other 
// applications send host names like "www.jonasjohn.de:80" 
$host = str_replace(':80', '', $host);
$host = trim($host);
 
// if the host is not starting with www. redirect the 
// user to the same URL but with www :-)
if ($host != 'www.jonasjohn.de'){
    // You an also change the "!=" to "==", if you want to force 
    // the user to use the domain name without the www. 
 
    // send status header, so that search engines or other services
    // detect that this is a permanent redirect and not a temporary
    header('HTTP/1.1 301 Moved Permanently');
 
    // read the URL the user requested:
    $url = isset($_SERVER["REQUEST_URI"]) ? $_SERVER["REQUEST_URI"] : '';
 
    // redirect the user to the new destination:
    header('Location: http://www.jonasjohn.de' . $url);
 
    // Convert "special" chars -- cause we never now... ;-)
    $url = htmlspecialchars($url);
 
    // "fallback" link, if the browser is not supporting header redirects
    print '<a href="http://www.jonasjohn.de' . $url.'">Please click here</a>';
 
    // stop the script execution here
    exit;
}
 
// If the domain is www.jonasjohn.de then go on with your PHP code 
// of with your website...
 
// BTW: You need to replace jonasjohn.de trough your own domain :-D