Very small implementation of Google's OTP Authenticator

mini_google_authenticator.php

<?php
// copied from python code at https://stackoverflow.com/a/23221582/3103058
function base32_decode($key) {
  // https://www.php.net/manual/en/function.base-convert.php#122221
  $key = strtoupper($key);
  list($t, $b, $r) = array("ABCDEFGHIJKLMNOPQRSTUVWXYZ234567", "", "");
  foreach(str_split($key) as $c)
    $b = $b . sprintf("%05b", strpos($t, $c));
  foreach(str_split($b, 8) as $c)
    $r = $r . chr(bindec($c));
  return($r);
}
function Truncate($hmac_sha1){
  $offset = hexdec(substr($hmac_sha1, -1));
  $binary = hexdec(substr($hmac_sha1, $offset * 2, 8)) & 0x7fffffff;
  return $binary;
}
function HOTP($K, $C, $digits = 6) {
  $decodeK = base32_decode($K);
  //$C_bytes = pack('J', $C);
  $C_bytes = pack('N', 0) . pack('N', $C); //work with PHP version < 5.6.3
  $hmac_sha1 = hash_hmac("sha1", $C_bytes,$decodeK);
  return substr(Truncate($hmac_sha1), -$digits);
}
function TOTP($K, $digits = 6, $window = 30) {
  $C = (int)(time() / $window);
  return HOTP($K, $C, $digits);
}
//produce OTP for key: "MZXW633PN5XW6MZX"
echo TOTP("MZXW633PN5XW6MZX");