mb_convert_encoding( $str, $encoding1,$encoding2 );

$str,要转换编码的字符串
$encoding1,目标编码,如utf-8,gbk,大小写均可
$encoding2,原编码,如utf-8,gbk,大小写均可

实例1

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

实例2

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

实例3

<?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
?>