How to hide some characters in php

/**
 * 隐藏部分字符串
 * # 此方法多用于手机号码或身份证号、银行卡号的中间部分数字的隐藏
 */
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;
}

/**
 * 只保留字符串首尾字符，隐藏中间用*代替（两个字符时只显示第一个）
 * @param string $user_name 姓名
 * @return string 格式化后的姓名
 */
function substr_cut($user_name){
    $strlen   = mb_strlen($user_name, 'utf-8');
    $firstStr   = ucfirst(strtolower(mb_substr($user_name, 0, 3, 'utf-8')));
    $lastStr   = strtolower(substr($user_name, -3));
    if($strlen == 2){
        $hideStr = str_repeat('*', strlen($user_name, 'utf-8') - 1);
        $result = $firstStr . $hideStr ;
    }else {
        $hideStr = substr(str_repeat("*", $strlen - 6), 0, 3);
        $result = $firstStr . $hideStr . $lastStr;
    }
    return $result;
}