An example of using this to find all the code points in a string that cannot be transliterated to Latin-ASCII:

<?php

$string = "Народm, Intl gurus get paid €10000/hr 😁";

$latinAscii = Transliterator::create('NFC; Any-Latin; Latin-ASCII;');
$transliterated = $latinAscii->transliterate($string);

$codePoints = IntlBreakIterator::createCodePointInstance();
$codePoints->setText($transliterated);

foreach ($codePoints->getPartsIterator() as $char) {
    $ord = IntlChar::ord($char);
    if (255 < $ord) {
        echo IntlChar::charName($ord) . "\n";
    }
}
?>

Outputs:
EURO SIGN
GRINNING FACE WITH SMILING EYES