find path of a key in a multi-dimensional tree-like array

Programmer Question

hey, I have this array (the actual array can be several level deeps and spans a tree-structure)

array
  3 => 
    array
      4 => 
        array
          7 => null
          8 => null
      5 => null
  6 => null

Now, e.g. I want the path to key 7, it can be shown like this:

array
  0 => int 7
  1 => int 4
  2 => int 3

Can someone help me with such a recursion function?
I am able to get:

array
  0 => int 7
  1 => int 4

using:

function tree_path($key,$tree,&$path=null) {
    if(!is_array($path)) $path = array();
    if(is_array($tree)) {
        if (!array_key_exists($key,$tree)) {
            foreach ($tree as $id=>$child) {
                $found = tree_path($key,$child,$path);
                if($found) $path[] = $id;
            }
        } else {$path[] = $key; return true; }
    }
}

Oh. I recently saw the use of passing-by-reference in foreach loop. Can something like that be added here?

Find the answer here