function hotp($key,$c)	{
	$mask = ['f' => '7','e' => '6','d' => '5','c' => '4','b' => '3','a' => '2','9' => '1','8' => '0'];
	$hash_value_str = hash_hmac("sha1",pack("H*",sprintf("%016x",$c)),$key);
	$offset = intval($hash_value_str[39],16)*2;
	$extract = substr($hash_value_str,$offset,8);
	if(isset($mask[$extract[0]]))	{
		$extract[0] = $mask[$extract[0]];
	}
	$value = intval($extract,16);
	return ($value % 1000000);
}

function hotp($key,$c)	{
	$mask = ['f' => '7','e' => '6','d' => '5','c' => '4','b' => '3','a' => '2','9' => '1','8' => '0'];
	echo "key: $key\n";
	echo "c: $c\n";
	$hash_value_str = hash_hmac("sha1",$c,$key);
	echo "hash: $hash_value_str\n";
	echo "4bits (hex): ".$hash_value_str[39]."\n";
	$offset = intval($hash_value_str[39],16)*2;
	$extract = substr($hash_value_str,$offset,8);
	echo "extract: $extract\n";
	if(isset($mask[$extract[0]]))	{
		echo $extract[0]."& 7 =";
		$extract[0] = $mask[$extract[0]];
		echo $extract[0]."\n";
		echo "new extract: $extract\n";
	}
	$value = intval($extract,16);
	echo "Value: $value\n";
	return ($value % 1000000);
}

$base32 = new Base2n(5, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567', FALSE, TRUE, TRUE);
$ct = floor(time()/30);
$key = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
echo "Key in base32: ".$base32->encode($key)."\n";
 
printf("%06d\n",hotp($key,$ct));
printf("%06d\n",hotp($base32->encode($key),$ct));
 

Key in base32: IFAUCQKBIFAUCQKBIFAUCQKBIFAUCQKBIFAUCQKBIFAUCQKBIFAQ====
key: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
c: 52566234
hash: 428fc537d16c867ee7e1b4b3cc432c7e288d5769
4bits (hex): 9
extract: e1b4b3cc
e& 7 =6
new extract: 61b4b3cc
Value: 1639232460
232460
key: IFAUCQKBIFAUCQKBIFAUCQKBIFAUCQKBIFAUCQKBIFAUCQKBIFAQ====
c: 52566234
hash: a49ac9470db4a542a36b42d28b7e089e11123f8a
4bits (hex): a
extract: 42d28b7e
Value: 1121094526
094526

$ct = floor(time()/30);