php mb_convert_encoding問題

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

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

原因:不明な解決:現在「auto」を「gbk」に変更