this class seems to be painful: it is not, formatting and parsing are highly customizable, but what you probably need is really simple:

if you want to localize a number use:

<?php
$a = new \NumberFormatter("it-IT", \NumberFormatter::DECIMAL);
echo $a->format(12345.12345) . "<br>"; // outputs 12.345,12
$a->setAttribute(\NumberFormatter::MIN_FRACTION_DIGITS, 0);
$a->setAttribute(\NumberFormatter::MAX_FRACTION_DIGITS, 100); // by default some locales got max 2 fraction digits, that is probably not what you want
echo $a->format(12345.12345) . "<br>"; // outputs 12.345,12345
?>

if you want to print money use:

<?php
$a = new \NumberFormatter("it-IT", \NumberFormatter::CURRENCY);
echo $a->format(12345.12345) . "<br>"; // outputs €12.345,12
?>

if you have money data stored as (for example) US dollars and you want to print them using the it-IT notation, you need to use

<?php
$a = new \NumberFormatter("it-IT", \NumberFormatter::CURRENCY);
echo $a->formatCurrency(12345, "USD") . "<br>"; // outputs $ 12.345,00 and it is formatted using the italian notation (comma as decimal separator)
?>

another useful example about currency (how to obtain the currency name by a locale string):

<?php
$frontEndFormatter = new \NumberFormatter("it-IT", \NumberFormatter::CURRENCY);
$adminFormatter = new \NumberFormatter("en-US", \NumberFormatter::CURRENCY);
$symbol = $adminFormatter->getSymbol(\NumberFormatter::INTL_CURRENCY_SYMBOL); // got USD
echo $frontEndFormatter->formatCurrency(12345.12345,  $symbol) . "<br>";
?>

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

?>

Here's an example of how to use PATTERN_DECIMAL to print a number with two fraction digits, use () for negative numbers and pad to five characters to the left of the decimal point, using spaces as the padding character:

<?php

$fmt = new NumberFormatter("en-CA", NumberFormatter::PATTERN_DECIMAL, "* #####.00 ;(* #####.00)");
echo $fmt->format(-45.1);

// Outputs: "  (45.10)"

?>

Note that the ; in the pattern denotes the beginning of a subpattern, which is used for negative numbers. Hence the brackets around the pattern after the semicolon.

Sample script to print number in English.

<?php
$f = new NumberFormatter("en", NumberFormatter::SPELLOUT);
echo $f->format(123456);

?>

Produces the result:
one hundred twenty-three thousand four hundred fifty-six

Be aware that (at least with the locale 'fr-FR') NumberFormatter doesn't use spaces. It doesn't even use non breakable spaces (NBSP). It uses narrow non breakable spaces (NNBSP). This broke my tests.

<?php

$formatter = new NumberFormatter(
    'fr-FR',
    NumberFormatter::DEFAULT_STYLE
);

$value = $formatter->format(100_000); // '100 000'

// If you want to replace narrow non breakable spaces with non breakable spaces:

str_replace("\u{202F}", "\u{00A0}", $value);

// If you want to replace it with a normal space

str_replace("\u{202F}", " ", $value);