php 截取字符串固定长度，并用省略号代替未显示字符

<?php

function substring($str, $start, $len) {
     $tmpstr = "";
     $strlen = $start + $len;
     for($i = 0; $i < $strlen; $i++) {
         if(ord(substr($str, $i, 1)) > 0xa0) {
             $tmpstr .= substr($str, $i, 2);
             $i++;
         } else
             $tmpstr .= substr($str, $i, 1);
     }
 
     return $tmpstr."······";
 
}

echo substring('截取字符串固定长度',0,8);

?>