code: public function post($key = null, $filter = false)
      {
          if (is_null($key)) {
              return $_POST;
          }
  
          $value = (isset($_POST[$key]) ? $_POST[$key] : null);
  
          return $this->filter($value, $filter);
      }
docs: HTTP POST Request
  
  @param string $key
  @param bool   $filter
  
  @return string|null