<?php
    function isValid ($arg)
    {
        if (strlen ($arg) != 13)
            return false;

        $chk = false; //true isvalid false is invalid
        $str = strtoupper ($arg);
        $checkBar = true;

        if ($str{10} == "X")
        {
            $checkBar = false;
            $chk = true;
        }

        if ($checkBar)
        {
            $sum = 0; //to check barcode
            $sum += (int)$str{2}*8;
            $sum += (int)$str{3}*6;
            $sum += (int)$str{4}*4;
            $sum += (int)$str{5}*2;
            $sum += (int)$str{6}*3;
            $sum += (int)$str{7}*5;
            $sum += (int)$str{8}*9;
            $sum += (int)$str{9}*7;
            $sum %= 11;

            if ($sum == 0)
            {
                if ($str{10} == "5")
                    $chk = true;
                else
                    return false;
            }
            else
                if ($sum == 1)
                {
                    if ($str{10} == "0")
                        $chk = true;
                    else
                        return false;
                }
                else
                    if ((int)$str{10} == (11-$sum))
                        $chk = true;
                    else
                        return false;
        }

        switch ($str{0})
        {
            case "E" :
            case "C" :
            case "R" :
            case "L" : return true;
        }


        if (substr ($str,0,2) == "DS")
            return true;
        if ($str{0} == "V" && substr ($str,11,2) != "TH")
            return true;
        if (substr ($str,0,2) == "PE" && substr ($str,11,2) != "TH")
            return true;
        return false;
    }
?>