The NumberFormatter class can be used to convert integer numbers to Roman numerals without a custom function using an array of symbols and associated values:

<?php

function intToRomanNumeral(int $num) {
    static $nf = new NumberFormatter('@numbers=roman', NumberFormatter::DECIMAL);
    return $nf->format($num);
}

echo intToRomanNumeral(2); // II

echo intToRomanNumeral(5); // V

echo intToRomanNumeral(10); // X

echo intToRomanNumeral(50); // L

echo intToRomanNumeral(57); // LVII
echo intToRomanNumeral(58); // LVIII

echo intToRomanNumeral(100); // C

echo intToRomanNumeral(150); // CL

echo intToRomanNumeral(1000); // M

echo intToRomanNumeral(10000); // ↂ

?>