set elongated array keys

function test_set_elongated_keys() {
        $data = [
            'test' => 'value',
            'level_one' => [
                'level_two' => [
                    'level_three' => [
                        'replace_this_array' => [
                            'key_one' => 'testing',
                            'key_two' => 'value',
                            'special_key' => 'replacement_value',
                            'four' => 'another value'
                        ]
                    ],
                    'ordinary_key' => 'value'
                ]
            ]
        ];

        $completeIterator = new RecursiveIteratorIterator( new RecursiveArrayIterator( $data ) );
        $cfg = array();

        foreach ( $completeIterator as $key => $value ) {
            $current_depth = $completeIterator->getDepth();
            for ( $i = $current_depth - 1; $i >= 0; $i-- ) {
                $sub_iterator = $completeIterator->getSubIterator( $i );
                $key = $sub_iterator->key() . '.' . $key;
            }
            $cfg[$key] = $value;
        }

        $copy = $completeIterator->getArrayCopy();
    }