Arabic to Roman numerals converter

Converter.php

<?php

namespace App;

/**
 * Converts positive decimal numbers to Roman numerals.
 * Note: M (1000) is the largest Roman numeral supported by this function.
 */
class Converter
{
    protected $mappings = [
        [ 'I', 'V', 'X' ],
        [ 'X', 'L', 'C' ],
        [ 'C', 'D', 'M' ],
        //[ 'M', 'ↁ', 'ↂ'],
    ];

    function digit($num, $mapping)
    {
        if ($num <= 3) {
            return str_repeat($mapping[0], $num);
        }
        if ($num == 4) {
            return $mapping[0] . $mapping[1];
        }
        if ($num <= 8) {
            return $mapping[1] . str_repeat($mapping[0], $num - 5);
        }
        // if $num === 9
        return $mapping[0] . $mapping[2];
    }

    function convert($number) {
        $numString = "$number";
        $roman = '';
        $mappings = $this->mappings;

        while(strlen($numString) > 0 && !empty($mappings)) {
            $digit = substr($numString, -1);
            $numString = substr($numString, 0, -1);
            $currentMapping = array_shift($mappings);

            $roman = $this->digit((int)$digit, $currentMapping) . $roman;
        }

        $roman = str_repeat($currentMapping[2], (int)$numString) . $roman;

        return $roman;
    }
}