/ Published in: PHP
Basically what this algorithm does is to replace the first letter of the serial number by a number (each letter represents where the banknote is originally from), then it calculates the sum of all digits. If the remaining of the division of that sum by 9 is 0 then your note is valid.
URL: http://www.alixaxel.com/wordpress/2007/06/23/euro-banknote-validation-with-php/