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