class AppField
{
	private $local = ['code'=>null, 'title'=>null, 'description'=>null];
	private function _fail($name)
	{
		if (!array_key_exists($name, $this->local))
		{
			$bt = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 2)[1];
			trigger_error("Invalid property '{$name}' in {$bt['file']}:{$bt['line']}", E_USER_NOTICE);
			return true;
		}
	}
	public function __set($name, $value)
	{
		if ($this->_fail($name)) return;
		$this->local[$name] = $value;
	}
	public function __get($name)
	{
		if ($this->_fail($name)) return;
		return $this->local[$name];
	}
	public function __isset($name)
	{
		if ($this->_fail($name)) return;
		return isset($this->local[$name]);
	}
	public function __unset ($name)
	{
		if ($this->_fail($name)) return;
		$this->local[$name] = null;
	}
	public function __call($name, $arguments)
	{
		if ($this->_fail($name)) return;
		if (isset($arguments)) $this->local[$name] = $arguments[0];
		return $this->local[$name];
	}
}

class Field {
  public $name;
  public $value;
  function __construct($name, $value) {
    $this->name = $name;
    $this->value = $value;
  }
}

class Fields {
  private $list = [];
  function add($name, $value) { $this->list[$name] = new Field($name, $value); }
  function delete($name) { unset($this->list[$name]); }
...
}

class Sections {
  private $local = ['code'=>null, 'title'=>null, 'description'=>null];
  private $fields;
  ...
  function __construct() {
    $this->fields = new Fields;
  }
  ...
  public function __set($name, $value) {
	if ($name == 'Fields') {
		$bt = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 2)[0];
		trigger_error("Read-only property '{$name}' in {$bt['file']}:{$bt['line']}", E_USER_NOTICE);
		return;
	}
	if ($this->_fail($name)) return;
	$this->local[$name] = $value;
  }
  public function __get($name) {
	if ($name == 'Fields') {
		return $this->Fields;
	}
	if ($this->_fail($name)) return;
	return $this->local[$name];
  }
...
}