code: public function f($x)
      {
          if (!is_numeric($x)) {
              throw new \InvalidArgumentException('x variable must be numeric value.');
          }
  
          $float_fx = exp(-0.5 * pow(($x - $this->float_mu) / $this->float_sigma, 2)) / ($this->float_sigma * sqrt(2 * pi()));
  
          if ($this->int_precision) {
              return round($float_fx, $this->int_precision);
          }
  
          return $float_fx;
      }
docs: Function that returns normal distribution value for given x.
  
  @see NormalDistribution::precision()
  @throw \InvalidArgumentException If x is not numerical value.
  @param  float $x
  @access public
  @return float