PHP NumberFormatter add grouping for 5+ digit numbers?

I'm trying to fix a formatting error in the PHP NumberFormatter class for my language, Bulgarian: thousands grouping is only introduced for 5+ digit numbers, i.e. "5 000" is wrong.

Is there a predefined constant I can use?

I thought this would be NumberFormatter::GROUPING_SIZE but this doesn't take into consideration the above rule. Should I be looking for some other attribute or is what I'm trying to do not possible?

While it's not likely to be able to do this directly, you can use a wrapper class that modifies the output to what you expect:

class BulgarianNumberFormatter extends \NumberFormatter
{
    public function __construct(
        string|null $locale = 'bg_BG',
        int $style = null,
        string $pattern = null
    ) {
        parent::__construct($locale, $style, $pattern);
    }

    public function format(
        int|float $num,
        int $type = self::TYPE_DEFAULT
    ): string|false {
        $numOfWholeDigits = $num != 0 ? floor(log10($num) + 1) : 1;
        $formatted = parent::format($num, $type);
        if($numOfWholeDigits < 6) {
            $formatted = str_replace(
                $this->getSymbol(static::GROUPING_SEPARATOR_SYMBOL),
                '',
                $formatted
            );
        }
        return $formatted;
    }
}

To use:

require_once('/path/to/BulgarianNumberFormatter.php');
// or
use Namespace\Of\BulgarianNumberFormatter;

$formatter = new BulgarianNumberFormatter(style:\NumberFormatter::DECIMAL);

$formatter->format(5000);       // 5000
$formatter->format(120500);     // 120 500
$formatter->format(10005000);   // 10 005 000
$formatter->format(3210005000); // 3 210 005 000

As a note, the INTL extension is not run directly by PHP. It is a wrapper for the ICU library that is available on just about every system and used for formatting & translation - even by web browsers. If you think this should be changed in the ICU library, you can request changes here: https://icu.unicode.org/bugs