<?php define('WEIGHTS', [7, 3, 1]); define('WEIGHTS_COUNT', count(WEIGHTS)); $customerNumber = 1; $invoiceNumber = 25; echo ($refBase = sprintf('%d%04d', $customerNumber, $invoiceNumber)) . $verificationDigit = ( fn($weightedSum) => (ceil($weightedSum / 10) * 10 - $weightedSum) )(array_reduce( str_split(strrev($refBase)), function($result, $v) { static $i; $i ??= 0; return $result + ($v * WEIGHTS[$i++ % WEIGHTS_COUNT]); }, 0 ) );
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