php mb_convert_encoding 問題

問題：Warning: mb_convert_encoding() [function.mb-convert-encoding]: Unable to detect character encoding

<!-- lang: php -->
<?php
header("Content-Type:text/html;charset=utf8");
$str = "我是中國人";//gbk文件
echo mb_convert_encoding($str, "UTF-8", "auto");
?>

緣由：未知 解決：目前把"auto"換成"gbk"