Code vat check

Validation.php

/**
* Custom validation for the customer_profile_billing checkout pane.
*/
function arkaos_glue_commerce_checkout_form_validate($form, &$form_state, $checkout_pane, $order) {
  $valid = TRUE;
  
  // check vat
  $vat = $form_state['values']['customer_profile_company_info']['field_vat_number']['und'][0]['vat_number'];
  $country = $form_state['values']['customer_profile_billing']['commerce_customer_address']['und'][0]['country'];
  
  if(!empty($vat) && strtoupper(substr($vat,0,2)) !== $country) {
    form_set_error('customer_profile_company_info][field_vat_number][und][0][vat_number',t('This VAT number does not match the billing address you provided.'));
    $valid = FALSE;
  }
  
  // Function must return TRUE or FALSE, not just set a form error.
  // Again, see http://drupal.org/node/1796488
  return $valid;
}