public function validate($vatNumber)
{
if (strlen($vatNumber) != 11) {
return false;
}
$product = 10;
for ($i = 0; $i < 10; $i++) {
$sum = ($vatNumber[$i] + $product) % 10;
$sum = $sum == 0 ? 10 : $sum;
$product = 2 * $sum % 11;
}
return ($product + (int) $vatNumber[10]) % 10 == 1 ? true : false;
}