Расчет UPC-кода

/*///////////////////////////////////////////////////////////////

 Calculate UPC (UPC-A), PHP, 2010-12-10
 
 Artix, master@7masterov.ru, icq:53666599, skype:artixmaster
 
 * Error in code? Nothing is perfect!
 * Free source for free Linux, use it for free!
 * Please, do not remove this comment!

///////////////////////////////////////////////////////////////*/

function genUPC() {
    $code = ''; while(strlen($code)<11) $code.=rand(0,9);
    $s1 = 0; $s2 = 0;
    for($i=0;$i<strlen($code);$i+=2) {
        $s1+=intval($code{$i});
        if (isset($code{$i+1})) $s2+=intval($code{$i+1});
    }
    $s1*=3;
    $r = (10-($s1+$s2) % 10) % 10;
    $s=$code.$r;
    return $s;
}

function checkUPC($code) {
    if (strlen($code)==12) {
        $s1=0; $s2=0;
        for($i=0;$i<strlen($code);$i+=2) {
            $s1+=intval($code[$i+1]);
            $s2+=intval($code[$i]);
        }
        $s2*=3;
        $check = ($s1+$s2) % 10;
        if ($check==0) return true;
    }
    return false;
}

$x = genUPC();
echo checkUPC($x)? "$x correct\n" : "$x incorrect!\n";