PHP multi dimensional array keys into one single array?

For this particular application, I would use a class instead of arrays. The reason I say that is that it will be much easier to handle multiple levels, since you can have recursive functions in the class to access it. Here's a general example of what I'm talking about:

class Node {
    public $children = array();
    public $key;

    public function get_element_by_key($search_key){
        if($this->key == $search_key) return $this;
        foreach ($this->children as &$child) {
            if($child->get_element_by_key($search_key) != NULL){
                return $child;
            }
            else return NULL;
        }
    }
}

You could also add functions to add elements, a constructor to create an element and add to a parent and so on. Then you'd use the function get_element_by_key() (or similar) to access the elements. By the way, I haven't tried this code, I just wrote it to make a point. I bet you could get it to work though!

$ritit = new RecursiveIteratorIterator(new RecursiveArrayIterator($myArray), RecursiveIteratorIterator::SELF_FIRST);
$keys = array();
foreach ($ritit as $leafValue) {
    foreach (range(0, $ritit->getDepth()) as $depth) {
        $keys[] = $ritit->getSubIterator($depth)->key();
    }
}
print_r(array_unique($keys));

i added the RecursiveIteratorIterator::SELF_FIRST flag for good measure, although its not required in this case(the case that all sub arrays have at least one element). In fact, for this case, it would be more efficient without the flag.

I'm not sure I understand you question entirely. If you want to store multi-dimensional keys in a single dimensional array you'll need some way to encode them. Then you can walk the entire "deep" structure and for each leaf node add the encoded key to a list.

If you just want to duplicate the array, the same basically applies. You'll need to traverse the array, building the copy as you go, or use a deep copy function such as discussed at Deep copy of PHP array of references

The piece you may be missing is that a multi-dimensional array is just an array of arrays. Good stuff in oreilly's php book