Generate vancode.
Consists of a leading character indicating length, followed by N digits with a numerical value in base 36. Vancodes can be sorted as strings without messing up numerical order.
It goes: 00, 01, 02, ..., 0y, 0z, 110, 111, ... , 1zy, 1zz, 2100, 2101, ..., 2zzy, 2zzz, 31000, 31001, ...
function int2vancode($i = 0) {
$num = base_convert((int) $i, 10, 36);
$length = strlen($num);
return chr($length + ord('0') - 1) . $num;
}