PHP - Find parent key of array

Question

I'm trying to find a way to return the value of an array's parent key.

For example, from the array below I'd like to find out the parent's key where $array['id'] == "0002". The parent key is obvious because it's defined here (it would be 'products'), but normally it'd be dynamic, hence the problem. The 'id' and value of 'id' is known though.

    [0] => Array
        (
            [data] => 
            [id] => 0000
            [name] => Swirl
            [categories] => Array
                (
                    [0] => Array
                        (
                            [id] => 0001
                            [name] => Whirl
                            [products] => Array 
                               (
                                    [0] => Array
                                        (
                                            [id] => 0002
                                            [filename] => 1.jpg
                                         )
                                    [1] => Array
                                        (
                                            [id] => 0003
                                            [filename] => 2.jpg
                                         )
                                )
                         )
                 )
          )

Answer 1

Since you have a tree structure either of a BFS or DFS can do it. Since the structure is variable a recursive solution would work well. Simply return a sentinel when you find the value, then return the key in the caller.

Answer 2

A little crude recursion, but it should work:

function find_parent($array, $needle, $parent = null) {
    foreach ($array as $key => $value) {
        if (is_array($value)) {
            $pass = $parent;
            if (is_string($key)) {
                $pass = $key;
            }
            $found = find_parent($value, $needle, $pass);
            if ($found !== false) {
                return $found;
            }
        } else if ($key === 'id' && $value === $needle) {
            return $parent;
        }
    }

    return false;
}

$parentkey = find_parent($array, '0002');