本文实例讲述了php强制用户转向www域名的方法。分享给大家供大家参考。具体分析如下：
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jb51.net会直接转向www.jb51.net，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.jb51.net" but some other
// applications send host names like "www.jb51.net: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.jb51.net‘){
// 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.jb51.net‘ . $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.jb51.net‘ . $url.‘">Please click here</a>‘;
// stop the script execution here
exit;
}
// If the domain is www.jb51.net then go on with your PHP code
// of with your website...
// BTW: You need to replace jb51.net trough your own domain :-D
希望本文所述对大家的php程序设计有所帮助。
原文：http://www.jb51.net/article/68183.htm