跟踪TCP包信息也许能做到。

没法识别，所有http头部信息都是可以篡改的

 /**
 * 是否使用了代理?
 *
 * @return  boolean
 */
function is_use_proxy()
{
    $via = getenv("HTTP_VIA"); 
    $forwarded_for = getenv("HTTP_X_FORWARDED_FOR"); 
    $xroxy_connection = getenv("HTTP_XROXY_CONNECTION"); 
    $pragma = getenv("HTTP_PRAGMA"); 

    return ($via == "") 
            && ($forwarded_for == "") 
            && ($xroxy_connection == "") 
            && ($pragma == "");
}