Damm Algorithm

damm.php

<?php

function Damm($string, $encode = true)
{
	if ($encode > 0)
	{
		$encode += 1;
		$string = str_split($string, 1);
		$matrix = array_chunk(str_split('0317598642709215486342068713591750983426612304597836742095815869720134894536201794386172052581436790', 1), 10);

		while (--$encode > 0)
		{
			$interim = 0;

			foreach ($string as $value)
			{
				$interim = $matrix[$interim][intval($value)];
			}

			$string[] = $interim;
		}

		return implode('', $string);
	}

	else if (strcmp($string, Damm(substr($string, 0, max(1, abs($encode)) * -1), max(1, abs($encode)))) === 0)
	{
		return substr($string, 0, max(1, abs($encode)) * -1);
	}

	return false;
}

http://en.wikipedia.org/wiki/Damm_algorithm