VatValidator.php

<?php
function is_valid_regex($vat,$country){
	// current regex function
	return true;
}

function validate_vat($vat,$country){

	if(is_valid_regex($vat,$country)) {

		$iso_code_2_data = array(
			'AT' => 'AT', //Austria
			'BE' => 'BE', //Belgium
			'BG' => 'BG', //Bulgaria
			'DK' => 'DK', //Denmark
			'FI' => 'FI', //Finland
			'FR' => 'FR', //France
			'FX' => 'FR', //France métropolitaine
			'DE' => 'DE', //Germany
			'GR' => 'EL', //Greece
			'IE' => 'IE', //Irland
			'IT' => 'IT', //Italy
			'LU' => 'LU', //Luxembourg
			'NL' => 'NL', //Netherlands
			'PT' => 'PT', //Portugal
			'ES' => 'ES', //Spain
			'SE' => 'SE', //Sweden
			'GB' => 'GB', //United Kingdom
			'CY' => 'CY', //Cyprus
			'EE' => 'EE', //Estonia
			'HU' => 'HU', //Hungary
			'LV' => 'LV', //Latvia
			'LT' => 'LT', //Lithuania
			'MT' => 'MT', //Malta
			'PL' => 'PL', //Poland
			'RO' => 'RO', //Romania
			'SK' => 'SK', //Slovakia
			'CZ' => 'CZ', //Czech Republic
			'SI' => 'SI'  //Slovania
		);

		if (array_search(substr($vat, 0, 2), $iso_code_2_data)) {
			$vat = str_replace(' ','', substr($vat, 2));
		}

		if(class_exists('SoapClient')) {

			$remoteCheckVatServiceWSDL = "http://ec.europa.eu/taxation_customs/vies/checkVatService.wsdl";
			$localCheckVatServiceWSDL = false;

			if(function_exists('wp_upload_dir')){
				$localCheckVatServiceWSDL = wp_upload_dir() . '/taxation_customs/vies/checkVatService.wsdl';
				if(!is_file($localCheckVatServiceWSDL)){
					$current = file_get_contents($remoteCheckVatServiceWSDL);
					file_put_contents($localCheckVatServiceWSDL, $current);
				}
			}
			if(!is_file($localCheckVatServiceWSDL)) {
				$localCheckVatServiceWSDL = $remoteCheckVatServiceWSDL;
			}
			$client = new \SoapClient( $localCheckVatServiceWSDL );
			$result = $client->checkVat( [
				'countryCode' => $country ,
				'vatNumber'   => $vat,
			] );
			return $result->valid;
		} else {

			if (array_key_exists($country, $iso_code_2_data)) {
				$response = file_get_contents('http://ec.europa.eu/taxation_customs/vies/viesquer.do?ms=' . $iso_code_2_data[$country] . '&iso=' . $iso_code_2_data[$country] . '&vat=' . $vat);
				if (preg_match('/\bvalid VAT number\b/i', $response)) {
					return true;
				}
				if (preg_match('/\binvalid VAT number\b/i', $response)) {
					return false;
				}
			}
		}
	}
	return false;
}

$vat = 'GB864438791';
$country = 'GB';

var_dump( validate_vat($vat,$country));die;