How to prevent NumberFormatter to round decimal numbers?

Question

I am trying to write a general purpose function for formatting decimal numbers that works with any number of decimals — honoring the original number of decimals and current locale (which can be set to any that the local enviroment supports).

This is what I have (in a test):

setlocale(LC_MESSAGES, 'en_US');

$formatter = new \NumberFormatter('en_US', \NumberFormatter::DECIMAL);

$num = 1234.56789;

$this->assertEquals('1,234.56789', $formatter->format($num, \NumberFormatter::TYPE_DOUBLE));

... but it will fail:

Failed asserting that two strings are equal.
Expected :'1,234.56789'
Actual   :'1,234.568'

By adding this line:

echo 'MAX_FRACTION_DIGITS: '. $formatter->getAttribute(\NumberFormatter::MAX_FRACTION_DIGITS) . PHP_EOL;

... I was able to determine that by default NumberFormatter rounds the number to 3 digits.

Setting a large number for MAX_FRACTION_DIGITS like so:

$formatter->setAttribute(\NumberFormatter::MAX_FRACTION_DIGITS, 100);

... will get me what I want, but I am sure I am missing something.

Is there a less hacky way to do what I want?