<?php

class MyArrayObject extends \ArrayObject
{
    public function offsetGet($index)
    {
        if (!parent::offsetExists($index)) {
            parent::offsetSet($index, new MyArrayObject(array(), \ArrayObject::ARRAY_AS_PROPS));
        }
        
        return parent::offsetGet($index);
    }

    // this magic method not called on non-exist property access 
    public function __get($index)
    {
        return $this->offsetGet($index);
    }
}


$arrObj1 = new \MyArrayObject(array(), \ArrayObject::ARRAY_AS_PROPS);


// error: undefined index foo BP_VAR_RW
$arrObj1->foo->bar->baz = 99;

// OK
$bar = $arrObj1->foo->bar;
$bar->baz = 99;

// OK
$arrObj1['foo']['bar']['baz'] = 99;


class OtherArrayObject implements \ArrayAccess
{
    private $storage = array();
    
    
    public function offsetExists($offset)
    {
        return isset($this->storage[$offset]);
    }
    
    public function offsetGet($offset)
    {
        if (!$this->offsetExists($offset)) {
            $this->offsetSet($offset, new OtherArrayObject());
        }
        
        return $this->storage[$offset];
    }
    
    public function offsetSet($offset, $value)
    {
        $this->storage[$offset] = $value;
    }
    
    public function offsetUnset($offset)
    {
        unset($this->storage[$offset]);
    }

    public function __get($name)
    {
        return $this->offsetGet($name);
    }
    
    public function __set($name, $value)
    {
        return $this->offsetSet($name, $value);
    }
    
    public function __isset($name)
    {
        return isset($this->storage[$name]);
    }
    
    public function __unset($name)
    {
        unset($this->storage[$name]);
    }
}

$arrObj2 = new OtherArrayObject();

// OK:
$arrObj2->foo->bar->baz = 99;

// OK:
$arrObj2['foo']['bar']['baz'] = 99;

var_dump($arrObj2);