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Luhn Algorithm

Published in: PHP

URL: http://reusablecode.blogspot.com/2009/03/luhn-algorithm.html

For verifying credit card numbers.

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<?php
    /*
    Copyright (c) 2008, reusablecode.blogspot.com; some rights reserved.
    
    This work is licensed under the Creative Commons Attribution License. To view
    a copy of this license, visit http://creativecommons.org/licenses/by/3.0/ or
    send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California
    94305, USA.
    */
 
    // Luhn (mod 10) algorithm
    function luhn($input)
    {
        $sum = 0;
        $odd = strlen($input) % 2;
 
        // Remove any non-numeric characters.
        if (!is_numeric($input))
        {
            eregi_replace("\D", "", $input);
        }
 
        // Calculate sum of digits.
        for($i = 0; $i < strlen($input); $i++)
        {
            $sum += $odd ? $input[$i] : (($input[$i] * 2 > 9) ? $input[$i] * 2 - 9 : $input[$i] * 2);
            $odd = !$odd;
        }
 
        // Check validity.
        return ($sum % 10 == 0) ? true : false;
    }
 
    // Unit test - expected result is true.
    echo (luhn("446667651")) ? "true" : "false";
?>

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