Multiply 2 numbers

multiply-in-column.php

function multiplyInColumn($a, $b) {

    $bArr = array_reverse(str_split($b));
    $aArr = array_reverse(str_split($a));

    $result = array_map(function ($step, $bItem) use ($aArr) {
        $rest = 0;
        $result = array_fill(0, $step, 0);

        foreach ($aArr as $key => $aItem) {
            $r = ($bItem * $aItem) + $rest;
            if ($r < 10) {
                $result[] = $r;
                $rest = 0;
            } else {
                if ($key === (count($aArr) - 1)) {
                    $result[] = $r;
                    $rest = 0;
                } else {
                    $result[] = (int) substr($r, -1);
                    $rest = substr($r, 0, 1);
                }
            }
        }

        return array_reduce($result, function ($carry, $item) {
            return $item . $carry;
        });

    }, array_keys($bArr), $bArr);

    return array_sum($result);

}

echo multiplyInColumn(12345, 12) . PHP_EOL;
echo multiplyInColumn(11, 1) . PHP_EOL;
echo multiplyInColumn(10, 999) . PHP_EOL;