http://www.wechall.net/challenge/space/host_me/index.php$_SERVER['HTTP_HOST']是从http头的Host读的，所以我们需要把Host设成localhost
另一方面，我们需要正常访问网址，所以要用absoluteURI
GET http://www.wechall.net/challenge/space/host_me/index.php HTTP/1.1 Host: localhost
↧
HOST me
↧