Paid Memberships Pro - Require VAT

pmpro-require-vat.php

<?php
function pmprovat_required_vat_fields_submission( $value ) {
	global $pmpro_msg, $pmpro_msgt;
	$vat_number = sanitize_text_field( $_REQUEST['vat_number'] );
	if ( empty( $vat_number ) ) {
		$pmpro_msg  = 'VAT is required';
		$pmpro_msgt = 'pmpro_error';
	}
	return $value;
}
add_filter( 'pmpro_registration_checks', 'pmprovat_required_vat_fields_submission', 9 );