Convert decimal integer to roman numerals

roman_numerals.php

<?php

$input = 3486;
$expected = '';

function whatever(int $input)
{
    $map = [
        1 => 'I',
        5 => 'V',
        10 => 'X',
        50 => 'L',
        100 => 'C',
        500 => 'D',
        1000 => 'M',
    ];
    $rules = [
        [1000, 100],
        [500, 100],
        [100, 10],
        [50, 10],
        [10, 1],
        [5, 1],
        [1, 0],
    ];
    $result = '';
    foreach ($rules as $set) {
        $upper = $set[0];
        $lower = $set[1];
        $upperChar = $map[$upper];
        $lowerChar = $map[$lower];
        $count = (int)($input / $upper);
        $input -= $count * $upper;
        $result .= str_repeat($upperChar, $count);
        $threshold = $upper - $lower;
        if ($input >= $threshold) {
            $input -= $threshold;
            $result .= $lowerChar . $upperChar;
        }
    }
    return $result;
}

echo $input . ' -> ' . whatever($input);