give a secret and a time and its return the current TOTP.

lib.otp.php

<?php

function getOTP($secret, $time) {

  $counter = floor($time/30);

  $arr_map = array_flip(str_split("ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"));
  $arr_secret = str_split(preg_replace('/[^A-Z2-7]/', '', strtoupper($secret)));
  
  $bin_secret = "";
  $n = 0;
  $j = 0;
  
  foreach($arr_secret as $chr) {
    $n = $n << 5;
    $n += $arr_map[$chr];
    $j += 5;
    if($j>=8) {
      $j -= 8;
      $bin_secret .= chr(($n & (0xFF << $j)) >> $j);
    }
  }
  
  $hash = hash_hmac ('sha1', pack('N*', 0).pack('N*', $counter), $bin_secret, true);
  $offset = ord($hash[19]) & 0xf;
  $otp = (
      ((ord($hash[$offset+0]) & 0x7f) << 24 ) |
      ((ord($hash[$offset+1]) & 0xff) << 16 ) |
      ((ord($hash[$offset+2]) & 0xff) << 8 ) |
      (ord($hash[$offset+3]) & 0xff)
  ) % pow(10, 6);
  
  return str_pad($otp, 6, "0", STR_PAD_LEFT);
}