La classe ArrayObject

As you know ArrayObject is not an array so you can't use the built in array functions. Here's a trick around that:

Extend the ArrayObject class with your own and implement this magic method:

<?php
    public function __call($func, $argv)
    {
        if (!is_callable($func) || substr($func, 0, 6) !== 'array_')
        {
            throw new BadMethodCallException(__CLASS__.'->'.$func);
        }
        return call_user_func_array($func, array_merge(array($this->getArrayCopy()), $argv));
    }
?>

Now you can do this with any array_* function:
<?php
$yourObject->array_keys();
?>
- Don't forget to ommit the first parameter - it's automatic!

Note: You might want to write your own functions if you're working with large sets of data.

There is a better explanation about the ArrayObject flags (STD_PROP_LIST and ARRAY_AS_PROPS) right here: 

http://stackoverflow.com/a/16619183/1019305

Thanks to JayTaph

If you need the last key of your collection use:
<?php
array_key_last($this->getArrayCopy())
?>

In an extending class it could look like:
<?php
class Collection extends ArrayObject
{
    public function lastKey(): int
    {
        return array_key_last($this->getArrayCopy());
    }
}
?>

If you want to use any type safe collection:
<?php
class BookCollection extends Collection
{
    public function add(Book $book) : void
    {
        $this->offsetSet($book->id, $book);
    }

    // note the return type "Book"
    public function get(int $bookId) : Book 
    {
        $this->offsetGet($bookId);
    }
}
?>

Generally variable $this can't be used as an array within an object context. For example, following code piece would cause a fatal error:

<?php
class TestThis {
    public function __set($name, $val