array_keys with objects

[Tamlyn Rhodes]

I have a gallery object which contains an array of image objects. I
would like the image objects to be able to return their adjacent
siblings in the array so that, for example,
$gallery->images[3]->nextImage() would return $gallery->images[4]. The
images each contain a reference to their parent (i.e. the gallery
object) so I tried using array_keys(). This is a method of the image class:

function nextImage() {
$pos = array_keys($this->parent->images, $this);
return $this->parent->images[$pos[0]+1];
}

The array_keys call *should* find the current object ($this) in the
array and return an array containing a single value which is the index
of the current image in the images. But instead I get "Fatal error:
Nesting level too deep - recursive dependency?"

I suppose this has something to do with the cyclic references. Can
anyone suggest a way around this or any other way for an image to access
it's siblings?

Thanks,

T.