Austrian VAT Number Validation

at_vat.php

<?php

$test = 'ATU10223006';

$check_array = array_chunk(
	str_split(substr($test, 3, -1)),
	2
);

$checksum = (10 - (4 +
	array_sum(
		array_column(
			$check_array, 
			0
		)
	)
	+
	array_sum(
		array_map(function($v) {
			return $v * 2 % 10 + floor($v / 5);
		}, array_column(
			$check_array, 
			1
		)
	))
) % 10) % 10;

$checksum_valid = $checksum === intval(substr($test, -1));
echo 'Valid: ' . ($checksum_valid ? 'Yes' : 'No');