code: public function get($id)
      {
          if (isset($this->children[$id])) {
              return $this->children[$id];
          }
  
          throw new FormalException(
              "Unknown form field '$id' on form '" . get_called_class() . "'. Available fields are: " . implode(', ', array_keys($this->children))
          );
      }
docs: Get a form item
  
  @param string $id
  
  @return IFormField
  @throws FormalException