Zend Framework : validateur algorithme Luhn

<?php
/**
 * @see Zend_Validate_Abstract
 */
require_once 'Zend/Validate/Abstract.php';
 
class Wiip_Validate_Luhn extends Zend_Validate_Abstract
{
    /**
     * Validation failure message key for when the value fails the mod-10 checksum
     */
    const CHECKSUM = 'luhnChecksum';
 
    /**
     * Digits filter for input
     *
     * @var Zend_Filter_Digits
     */
    protected static $_filter = null;
 
    /**
     * Validation failure message template definitions
     *
     * @var array
     */
    protected $_messageTemplates = array(
        self::CHECKSUM => "Luhn algorithm (mod-10 checksum) failed on '%value%'"
    );
 
    /**
     * Defined by Zend_Validate_Interface
     *
     * Returns true if and only if $value follows the Luhn algorithm (mod-10 checksum)
     *
     * @param string $value
     * @return boolean
     */
    public function isValid($value)
    {
        $this->_setValue($value);
 
        if (null === self::$_filter) {
            /**
             * @see Zend_Filter_Digits
             */
            require_once 'Zend/Filter/Digits.php';
            self::$_filter = new Zend_Filter_Digits();
        }
 
        $valueFiltered = self::$_filter->filter($value);
 
        $length = strlen($valueFiltered);
 
        $sum = 0;
 
        for ($i = $length - 1; $i >= 0; $i--) {
            $digit = (int) $valueFiltered[$i];
            if (($i % 2) == 1) {
                // Rang pair, on multiplie par 2
                $digit *= 2;
		if ($digit > 9)	$digit -= 9;
            }
            $sum += $digit;
        }
 
        if (($sum % 10) != 0) {
            $this->_error(self::CHECKSUM, $valueFiltered);
            return false;
        }
 
        return true;
    }
}

$this->addElement(
    'text',
    'siren',
    array(
        'description' => '9 chiffres',
        'label' => 'SIREN :',
        'maxlength' => 9,
        'size' => 9,
        'validators' => array(
            array('StringLength', true, 9),
            array('Luhn', true)
        )
    )
);