"pass" means "no conversion":
See here:
http://php.net/manual/en/mbstring.configuration.php#ini.mbstring.strict-detection
mbstring.http_input = pass ; No conversion.
mbstring.http_output = pass ; No conversion
(PHP 4 >= 4.0.6, PHP 5, PHP 7)
mb_http_input — 检测 HTTP 输入字符编码
type设置的字符串指定了输入类型。 "G" 是 GET，"P" 是 POST，"C" 是 COOKIE，"S" 是 string，"L" 是 list，以及 "I" 是整个列表（将会返回 array）。 如果省略了 type，它将返回最后处理的一种输入类型。
每个 type 的字符编码名称。
如果 mb_http_input() 没有处理过任何指定的 HTTP 输入，它将返回 FALSE。
"pass" means "no conversion":
See here:
http://php.net/manual/en/mbstring.configuration.php#ini.mbstring.strict-detection
mbstring.http_input = pass ; No conversion.
mbstring.http_output = pass ; No conversion
As this does not seam to work (always returns "pass"). you can use
$encoding = mb_detect_encoding(urldecode($REQUEST_URI), 'UTF-8, UTF-7, ASCII, EUC-JP,SJIS, eucJP-win, SJIS-win, JIS, ISO-2022-JP, ISO-8859-1');
to detect the encoding of the url instead, similare steps could be taken to deteck cookies and post data although it would have to be done for each value.