gistfile1.php

<?php

class Collection implements IteratorAggregate, ArrayAccess, Countable
{
    // ...

    public function toJSON()
    {
        $return = array();

        foreach ($this->_array as $item)
        {
            if (method_exists($item, "toJSON"))
            {
                $return[$key] = $value->toJSON();
            }
            else
            {
                $return[$key] = $value;
            }
        }

        return json_encode($return);
    }

    // ...
}

class Item
{
    public function toJSON()
    {
        $return = array();

        foreach ($this as $key => $value)
        {
            if (method_exists($value, "toJSON"))
            {
                $return[$key] = $value->toJSON();
            }
            else
            {
                $return[$key] = $value;
            }
        }

        return json_encode($return);
    }
}

class Form extends Item
{
    public function getFields()
    {
        if (get_class($this->_fields) !== "Collection")
        {
            $this->_fields = new Collection($this->_fields);
        }

        return $this->_fields;
    }

    // ...

    public function __get($name)
    {
        if ($name === "fields")
        {
            return $this->getFields();
        }
    }
}

class Field extends Item
{
    public function getInputs()
    {
        if (get_class($this->_inputs) !== "Collection")
        {
            $this->_inputs = new Collection($this->_inputs);
        }

        return $this->_inputs;
    }

    // ...

    public function __get($name)
    {
        if ($name === "inputs")
        {
            return $this->getInputs();
        }
    }
}