respect-validation/library/Rules/Cnpj.php

<?php
namespace Respect\Validation\Rules;

class Cnpj extends AbstractRule
{
    public function validate($input)
    {
        //Code ported from jsfromhell.com
        $c = preg_replace('/\D/', '', $input);
        $b = array(6, 5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2);

        if (strlen($c) != 14) {
            return false;
        }

        for ($i = 0, $n = 0; $i < 12; $n += $c[$i] * $b[++$i]);

        if ($c[12] != ((($n %= 11) < 2) ? 0 : 11 - $n)) {
            return false;
        }

        for ($i = 0, $n = 0; $i <= 12; $n += $c[$i] * $b[$i++]);

        if ($c[13] != ((($n %= 11) < 2) ? 0 : 11 - $n)) {
            return false;
        }

        return true;
    }
}