PHP - Access an array using another array as the key

I'm creating a class that assists in building settings pages for WordPress plugins. The developer instantiates the class by passing in an array of settings. A basic example looks like:

Array
(
    [textbox1] => Array
        (
            [title] => Textbox
            [type] => text
            [section] => general
            [desc] => The text description
        )

    [a_nice_dropdown] => Array
        (
            [title] => Select me
            [type] => select
            [section] => general
            [desc] => The select description
            [choices] => Array
                (
                    [red]   => Red
                    [blue]  => Blue
                    [green] => Green
                )
        )
)

This works fine. My class builds the options page and the inputs have HTML that looks like:

<input id="textbox1" type="text" name="options_slug[textbox1]" value="">

When "Save Changes" is clicked, my class grabs all the options tied to "options_slug" and stores them in a single wp_options entry as a nice serialized array, making it easy to grab later.

The New Challenge

I have a new project which requires multiple nested "repeater" fields, similar to the way Advanced Custom Fields handles it. I've created a new field type, to handle this, which can support "subfields". An example config output (from error_log) looks like:

Array
(
    [subfields_container] => Array
        (
            [title] => Subfields
            [type] => subfields
            [section] => general
            [desc] => This is the subfields description text
            [subfields] => Array
                (
                    [textbox2] => Array
                        (
                            [title] => Textbox
                            [type] => text
                            [section] => general
                            [desc] => The text description
                        )

                    [select1deep] => Array
                        (
                            [title] => Subfield Select
                            [type] => select
                            [choices] => Array
                                (
                                    [1] => 1
                                    [2] => 2
                                    [3] => 3
                                )

                            [std] => 1
                        )
                )
        )
)

I've configured the HTML output so an input inside a "subfields" container now looks like:

<input id="textbox1" type="text" name="options_slug[subfields_container][textbox2]" value="">

The idea being that the end user can easily group fields: i.e.,

$options = get_option('options_slug');

foreach($options['subfield_container'] as $subfield) {

    // etc...

}

The Problem

As I iterate through these multidimensional arrays, I need to continually update a $options variable at the current index so it can be saved to the DB. So where previously I was able to do:

$id = 'textbox1';
$options[$id] = $_POST['textbox1'];

Now I'm doing something like:

$id = array('subfields_container' => 'textbox2');
$options[$id] = $_POST['textbox2'];

But I get "illegal offset type" errors. Because I can't set an array property using another array.

I've considered just putting dashes in the ID's instead of creating a hierarchical array, something like:

<input id="textbox1" type="text" name="options_slug[subfields_container-textbox2]" value="">

But then I'll lose the ability to foreach over a specific part of the stored options.

The Question

What's the best way to dynamically set a value inside of a multidimensional array when the array is not fixed in structure?

Thank you

via Chebli Mohamed