A class to generate compliant "ulid"

Ulid.php

<?php

class Ulid
{
    private static $time = -1;
    private static $rand = [];

    public function generate(bool $lowercase = false)
    {
        if (\PHP_INT_SIZE !== 8) {
            throw new \RuntimeException("Ulid::generate() requires a 64-bit PHP runtime." );
        }

        $time = (int) (1000 * microtime(true));

        if ($time !== self::$time) {
            self::$rand = unpack('Jhigh/Jlow', "\0\0\0".substr_replace(random_bytes(10), "\0\0\0", 5, 0));
            self::$time = $time;
        } elseif (1<<40 === ++self::$rand['low']) {
            self::$rand['low'] = 0;

            if (1<<40 === ++self::$rand['high']) {
                self::$rand['low'] = self::$rand['high'] = (1<<40) - 1;
                usleep(1000);

                return self::generate($lowercase);
            }
        }

        return strtr(
            str_pad(base_convert($time, 10, 32), 10, '0', STR_PAD_LEFT)
            .str_pad(base_convert(self::$rand['high'], 10, 32), 8, '0', STR_PAD_LEFT)
            .str_pad(base_convert(self::$rand['low'], 10, 32), 8, '0', STR_PAD_LEFT)
        , 'abcdefghijklmnopqrstuv', $lowercase ? 'abcdefghjkmnpqrstvwxyz' : 'ABCDEFGHJKMNPQRSTVWXYZ');
    }
}