Example PHP script to decode a KWZ filename

decode_kwz_filename.php

<?php

function decode_filename(string $filename)
{
  $bytes = base32_decode($filename);
  // Convert to byte string
  $bin = join('', array_map('chr', $bytes));
  // Unpack data
  // Hex FSID (9 bytes, 18 chars) | Creation timestamp (uint32) | Modified timestamp (uint32)
  return unpack('H18fsid/Vcreated/Vmodified', $bin);
}

function base32_decode(string $input)
{
  // KWZ base32 alphabet
  $alphabet = 'cwmfjordvegbalksnthpyxquiz012345';
  $inputLength = strlen($input);
  // Create zerofilled output buffer
  $resultSize = ceil($inputLength / 8) * 5;
  $result = array_fill(0, $resultSize, 0);
  $resultPtr = 0;
  // Decoder state
  $shift = 8;
  $carry = 0;

  for ($i = 0; $i < $inputLength; $i++)
  {
    $char = $input[$i];
    // Ignore padding
    if ($char === '=')
      continue;
    // Check char is in alphabet
    else if (!str_contains($alphabet, $char))
      throw new Error('Invalid character');
    // Decode
    $code = strpos($alphabet, $char);
    $shift -= 5;
    if ($shift > 0)
      $carry |= $code << $shift;
    else if ($shift < 0)
    {
      $result[$resultPtr++] = $carry | ($code >> -$shift);
      $shift += 8;
      $carry = ($code << $shift) & 0xff;
    } 
    else
    {
      $result[$resultPtr++] = $carry | $code;
      $shift = 8;
      $carry = 0;
    }
  }
  // Trim
  return array_slice($result, 0, $resultPtr);
}