如果一个网站同一套程序绑定了多个域名 www.a.com ,www.b.com,为了区分他们的robots.txt、
百度站长验证文件等，如 www.a.com/robos.txt 的内容来自于 /Virtual/www.a.com/robots.txt 
www.b.com/robots.txt 的内容来自于 /Virtual/www.b.com/robots.txt 的方法：

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([a-zA-Z0-9_]{2,32}\.(xml|html|txt)) seo.php?file=$1 [L]

文件 /seo.php
<?php

/**
 * @param string $msg
 */
function _404($msg = ‘‘)
{
    header(‘HTTP/1.1 404 Not Found‘);
    header(‘Status: 404 Not Found ‘);    // 确保FastCGI模式下正常
    if (empty($msg)) $msg = ‘ 404 Not Found. ‘;
    exit($msg);
}

function getExt($file)
{
    return strrchr($file, ‘.‘);
}


$file = trim($_GET[‘file‘]);
if (!preg_match(‘/^[a-zA-Z0-9_]{3,32}\.(html|txt|xml)$/i‘, $file)) _404();
$dir = ‘Virtual/‘ . strtolower($_SERVER[‘HTTP_HOST‘]);

if (!is_dir($dir)) _404();
$filePath = $dir . ‘/‘ . $file;

if (file_exists($filePath) && is_file($filePath)) {

    switch (getExt($file)) {
        case ‘.txt‘:
            $type = ‘text/plain; charset=UTF-8‘;
            break;
        case ‘.xml‘:
            $type = ‘application/xml‘;
            break;
        case ‘.html‘:
        default:
            $type = ‘text/html; charset=UTF-8‘;
            break;
    }


    if (!empty($type)) {
        header(‘Content-type:‘ . $type);
    }

    echo file_get_contents($filePath);
} else {
    _404();
}

PHP 重写读取文件，适用多域名的情况！