Расчет GRID-кода

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

 Calculate GRID, PHP, 2010-12-10

 GRID = A1 + 5 (Issuer Code element) + 10 (Release number) + 1 (Checkcode)

 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 genGRID($issuer='12345',$release='0123456789') {
    $codes = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $acodes = str_split($codes);
    $bcodes = array_flip($acodes);
    $newcode = "A1".$issuer.$release;
    $p = 36;
    $n = $newcode;
    for($j=1;$j<=strlen($n);$j++) {
        $s = $p%37+$bcodes[$n{$j-1}];
        $p = ($s%36==0?36:$s%36) * 2;
    }
    $s = 36-$p%37+1;
    return $n.$acodes[$s];
}

function checkGRID($code) {
    if (strlen($code)==18) {
        $issuer = substr($code,2,5);
        $release = substr($code,7,10);
        $check = genGRID($issuer,$release);
        return ($check==$code);
    }
    return false;
}

$x = genGRID();
echo checkGRID($x)? "$x correctn" : "$x incorrect!n";