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，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 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 // 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: https://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="https://www.jb51.net' . $url.'">Please click here</a>'; // stop the script execution here exit; } // If the domain is then go on with your PHP code // of with your website... // BTW: You need to replace jb51.net trough your own domain :-D