Complete solution (see it running):
function formatNum1000($num) {
$tail = $num % 1000;
$head = (int)($num / 1000);
$char1 = chr(ord('A') + (int)($head / 26));
$char2 = chr(ord('A') + ($head % 26));
return sprintf('%s%s%03d', $char1, $char2, $tail);
}
function formatNum999($num) {
$tail = (($num - 1 ) % 999) + 1;
$head = (int)(($num - $tail) / 999);
$char1 = chr(ord('A') + (int)($head / 26));
$char2 = chr(ord('A') + ($head % 26));
return sprintf('%s%s%03d', $char1, $char2, $tail);
}
$ns = array(1, 500, 999, 1000, 1998, 1999, 2000, 25974, 25975, 25999, 26000, 675324, 675999);
foreach($ns as $n) {
$formatted1000 = formatNum1000($n);
$formatted999 = formatNum999 ($n);
echo "Num: $n => $formatted1000 / $formatted999\n";
}
Note: you need to make sure that the input number is within the valid range (0...675999 when including 000-numbers, 1...675324 otherwise)
Note: answer revised, missed the point earlier that 000 is not allowed
result AB000for1000?