php怎么实现部分字符隐藏

// 隐藏部分字符串
function func_substr_replace($str, $replacement = '*', $start = 1, $length = 3)
{
 $len = mb_strlen($str,'utf-8');
 if ($len > intval($start+$length)) {
 $str1 = mb_substr($str,0,$start,'utf-8');
 $str2 = mb_substr($str,intval($start+$length),NULL,'utf-8');
 } else {
 $str1 = mb_substr($str,0,1,'utf-8');
 $str2 = mb_substr($str,$len-1,1,'utf-8'); 
 $length = $len - 2; 
 }
 $new_str = $str1;
 for ($i = 0; $i < $length; $i++) { 
 $new_str .= $replacement;
 }
 $new_str .= $str2;
 return $new_str;
}