编码问题

//一般情况下用 iconv，只有当遇到无法确定原编码是何种编码，或者iconv转化后无法正常显示时才用mb_convert_encoding 函数.
<?php
$str='编码';
echo mb_convert_encoding($str, "UTF-8", "auto"); //未知原编码，通过auto自动检测后，转换编码为utf-8
?>

<?php
$str='编码';
echo mb_convert_encoding($str, "UTF-8", "GBK"); //已知原编码为GBK，转换为utf-8
?> 

<?php
$str='编码';
echo mb_convert_encoding($str, "UTF-8"); //编码转换为utf-8
?> 

 iconv('GB2312', 'UTF-8', $str); //将字符串的编码从GB2312转到UTF-8