Question

I guess it is has something to do with ArrayAccess because $this and the objects in $this->products[$key] do implement ArrayAccess. However there is no magic __get or __set anywhere.

var_dump($this->products[$key]['selected_options'][$option_key]);
// Output: string(7) "Größe:S"

$this->products[$key]['selected_options'][$option_key] = "test";

var_dump($this->products[$key]['selected_options'][$option_key]);
// Output: string(7) "Größe:S"

Does someone have any idea what is wrong here?

Also note that this does work:

$this->products[$key]['selected_options'] = array($option_key => "test");
// Output: string(4) "test"

ArrayAccess for Products same for $this (Cart) but with $products instead of $data:

class Product implements ArrayAccess
{
    protected $data;

    /* **** ArrayAccess **** */
    public function offsetExists($offset) {
        return isset($this->data[$offset]);
    }

    public function offsetGet($offset) {
        return $this->data[$offset];
    }

    public function offsetSet($offset , $value) {
        $this->data[$offset] = $value;
    }

    public function offsetUnset($offset) {
        unset($this->data[$offset]);
    }
}

Answer 1

You need to return by reference inside offsetGet.

From the manual:

While direct modification triggers a call to ArrayAccess::offsetSet(), indirect modification triggers a call to ArrayAccess::offsetGet(). In that case, the implementation of ArrayAccess::offsetGet() must be able to return by reference, otherwise an E_NOTICE message is raised.

Note, however, that this only works with PHP >= 5.3.4