code: public static function fromPolar($length, $ap, $av)
    {
      return new static($length * cos($ap) * cos($av), $length * sin($ap) * cos($av), $length * sin($av));
    }
docs: Construct coordinate by the vector length and direction.
  
  @param $length the vector length
  @param $ap angle in XY plane (in radians)
  @param $av angle off XY plane (in radians)