php 实现纯汉子字符串倒置

<?php
/**
 * 字符串倒置
 * @param unknown $str
 */
header("Content-Type:text/html;charset=utf8");

function mystring($str){
	$arr=array();
	$len = mb_strlen($str,"UTF-8");
	for($i=0;$i<$len;$i++){
		$arr[]=mb_substr($str,$i,1,"UTF-8");
	}
	$arr=array_reverse($arr);
	return implode("", $arr);
}

echo mystring("字符串倒置");