private function waitUntilRequestQuotaAvailable()
{
    $sleep_until = (float)($this->currentStartTime + $this->intervalSeconds);
    $sleep_seconds = $sleep_until - microtime(true);

    // Avoid using time_sleep_until() as it appears to be less precise and not sleep long enough.
    usleep((int) $sleep_seconds * 1000000);

    // Ensure that enough time has passed as usleep() may not have waited long enough.
    $this->currentStartTime = microtime(true);
    if ($this->currentStartTime < $sleep_until) {
        do {
            usleep(1000000 / 4);
            $this->currentStartTime = microtime(true);
        } while ($this->currentStartTime < $sleep_until);
    }

    $this->currentRequestCount = 0;
}

private function waitUntilRequestQuotaAvailable()
{
    $sleep_until = (float)($this->currentStartTime + $this->intervalSeconds);
    $sleep_seconds = $sleep_until - microtime(true);

    // Avoid using time_sleep_until() as it appears to be less precise and not sleep long enough.
    // Avoid using usleep(): "Values larger than 1000000 (i.e. sleeping for
    //   more than a second) may not be supported by the operating system.
    //   Use sleep() instead."
    $sleep_seconds_int = (int)$sleep_seconds;
    if ($sleep_seconds_int >= 1) {
        sleep($sleep_seconds_int);
    }

    // Ensure that enough time has passed as usleep() may not have waited long enough.
    $this->currentStartTime = microtime(true);
    if ($this->currentStartTime < $sleep_until) {
        do {
            usleep(1000000 / 4);
            $this->currentStartTime = microtime(true);
        } while ($this->currentStartTime < $sleep_until);
    }

    $this->currentRequestCount = 0;
}

private function waitUntilRequestQuotaAvailable()
{
    $sleep_until = (float) ($this->currentStartTime + $this->intervalSeconds);
    $sleep_seconds = $sleep_until - microtime(true);

    usleep((int) ($sleep_seconds * 1000000));

    $this->currentStartTime = microtime(true);
    $this->currentRequestCount = 0;
}