/**
 * 将一个字符串部分字符用*替代隐藏
 * @param string    $string   待转换的字符串
 * @param int       $bengin   起始位置，从0开始计数，当$type=4时，表示左侧保留长度
 * @param int       $len      需要转换成*的字符个数，当$type=4时，表示右侧保留长度
 * @param int       $type     转换类型：0，从左向右隐藏；1，从右向左隐藏；2，从指定字符位置分割前由右向左隐藏；3，从指定字符位置分割后由左向右隐藏；4，保留首末指定字符串
 * @param string    $glue     分割符
 * @return string   处理后的字符串
 */
function hideStr($string, $bengin = 0, $len = 4, $type = 0, $glue = "@") {
    if (empty($string))
        return false;
    $array = array();
    if ($type == 0 || $type == 1 || $type == 4) {
        $strlen = $length = mb_strlen($string);
        while ($strlen) {
            $array[] = mb_substr($string, 0, 1, "utf8");
            $string = mb_substr($string, 1, $strlen, "utf8");
            $strlen = mb_strlen($string);
        }
    }
    switch ($type) {
        case 1:
            $array = array_reverse($array);
            for ($i = $bengin; $i < ($bengin + $len); $i++) {
                if (isset($array[$i]))
                    $array[$i] = "*";
            }
            $string = implode("", array_reverse($array));
            break;
        case 2:
            $array = explode($glue, $string);
            $array[0] = hideStr($array[0], $bengin, $len, 1);
            $string = implode($glue, $array);
            break;
        case 3:
            $array = explode($glue, $string);
            $array[1] = hideStr($array[1], $bengin, $len, 0);
            $string = implode($glue, $array);
            break;
        case 4:
            $left = $bengin;
            $right = $len;
            $tem = array();
            for ($i = 0; $i < ($length - $right); $i++) {
                if (isset($array[$i]))
                    $tem[] = $i >= $left ? "*" : $array[$i];
            }
            $array = array_chunk(array_reverse($array), $right);
            $array = array_reverse($array[0]);
            for ($i = 0; $i < $right; $i++) {
                $tem[] = $array[$i];
            }
            $string = implode("", $tem);
            break;
        default:
            for ($i = $bengin; $i < ($bengin + $len); $i++) {
                if (isset($array[$i]))
                    $array[$i] = "*";
            }
            $string = implode("", $array);
            break;
    }
    return $string;
}

$phone = '18866665555';
echo hideStr($phone,4,4);

1886****888

$phone = '我的网站名称为PHP博客！';
$str =  hideStr($phone,7,5);
print_r($str);

我的网站名称为*****！