PHP 如何将字符串拆分为单个字符

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style type="text/css">
      span{ width:40px; margin-left:10px; padding:3px; font-weight:bold; color:red; background-color:#efefef; text-align:center; border:1px #ccc solid;}
</style>
<?
$key = "测试分词abc123，,。.《<";
echo "当前要拆分的词：".$key."<br><br>";
$txt_arr = Fen_Ci_Arr($key);
$txt_num = count($txt_arr);
echo "该词的字符个数：".$txt_num."<br><br>";
echo "字符拆分结果为：";
for ( $i=0; $i<$txt_num; $i++ ){
      echo '<span>'.$txt_arr[$i]."</span>";
}
//【分词函数】
//$charset变量为网页编码，如"gb2312"或"utf-8"
function Fen_Ci_Arr($str,$charset="utf-8") {
$strlen=mb_strlen($str);
while($strlen){
$array[]=mb_substr($str,0,1,$charset);
$str=mb_substr($str,1,$strlen,$charset);
$strlen=mb_strlen($str);
}
return $array;
}
?>