MathPHP\Statistics\Descriptive::standardDeviation PHP Method

    public static function standardDeviation(array $numbers, bool $SD＋ = false)
    {
        if (empty($numbers)) {
            return null;
        }
        $n = count($numbers);
        return $SD＋ ? sqrt(self::populationVariance($numbers)) : sqrt(self::sampleVariance($numbers));
    }

 /**
  * Standard error of the mean (SEM)
  * The standard deviation of the sample-mean's estimate of a population mean.
  * https://en.wikipedia.org/wiki/Standard_error
  *
  *       s
  * SEₓ = --
  *       √n
  *
  * s = sample standard deviation
  * n = size (number of observations) of the sample
  *
  * @param array $X list of numbers (random variable X)
  *
  * @return float
  */
 public static function standardErrorOfTheMean(array $X) : float
 {
     $s = Descriptive::standardDeviation($X, Descriptive::SAMPLE);
     $√n = sqrt(count($X));
     return $s / $√n;
 }