<?php

function mascara_doc($doc)
{
    if (strlen($doc) === 18) {
         return substr_replace($doc, '***.***/****', 3, -3);
    }

    return substr_replace($doc, '***.***', 4, -3);
}

var_dump(mascara_doc('100.222.333.44'));
var_dump(mascara_doc('12.345.678/1000-55'));