http://httpd.apache.org/docs/current/mod/mod_rewrite.html
例如：
RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} ^Mozilla/5.0.*
# RewriteRule 当条件满足时，把url怎么改写 紧跟在RewriteCond后面
RewriteRule index.php index.m.php
RewriteCond %{HTTP_USER_AGENT} ^Lynx.*
RewriteRule index.php index.L.php
# 上面的条件都不满足时，执行下列这个改写规则
RewriteRule index.php index.b.php