Sample implementation Damm algorithm in PHP. Generating and Validating. http://en.wikipedia.org/wiki/Damm_algorithm

damm_algorithm.php

<?php

/**
 * The Damm check digit
 * Damm validation & generation code in PHP. 
 * Damm algorithm is a check digit algorithm that detects all single-digit errors 
 * and all adjacent transposition errors. Totally anti-symmetric quasigroup.
 *
 * For more information cf. http://en.wikipedia.org/wiki/Damm_algorithm
 *
 * @author Wuri Nugrahadi <[email protected]>
 */

if (! function_exists('taq')) {
  function taq($digits) {
		$taq_table = array(
			'0317598642', '7092154863', '4206871359', '1750983426', '6123045978',
			'3674209581', '5869720134', '8945362017', '9438617205', '2581436790');
		$interim = 0;

		foreach (str_split($digits) as $digit) {
			if (! preg_match('~\d~', $digit)) {
				return FALSE;
			}
			$interim = substr($taq_table[$interim], $digit, 1);
		}
		return $interim;
	}
}

if (! function_exists('calc_check_digit')) {
	function calc_check_digit($digits) {
		return $digits.taq($digits);
	}
}

if (! function_exists('is_check_digit_valid')) {
	function is_check_digit_valid($digits) {
		return taq($digits) == 0;
	}
}


// Testing
$src = 21081046;
$res = calc_check_digit($src);
echo $res.' : ';
if (is_check_digit_valid($res) == 1) 
	echo 'Valid';
else
	echo 'Invalid';

?>