php过滤字符串中重复的字符（包含中文）

<?php
header("Content-Type:text/html; Charset=utf-8");
function mb_str_split( $string ) {
    return implode("", array_unique(preg_split("/(?<!^)(?!$)/u", $string ))); 
} 

$string   = "过过过过过ggggg年年年nnnnnn啦啦啦啦啦啦lllll"; 
$charlist = mb_str_split( $string ); 


echo $charlist;