Luhn Algorithm

<?php
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    */
 
    // 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";
?>