function bc ($num) {
$chars = 'abcdefghijklmnopqrstuvwxyz';
$b = 26;

$i = 0;
while (true) {
        if (pow($b, ++$i) > $num) { $i--; break; }
}
for ($j = $i; $j >= 0; $j--) {

        $c = $num % pow($b, $j);
        $c2 = ($num - $c) / pow($b, $j);
        $s .= substr($chars, $c2, 1);
        $num = $num - $c2 * pow($b, $j);

}

return $s;

}