<?php
$array = array(
    1 => array(
        5 => array(
            3 => 'testvalue1'
        )
    ),
    2 => array(
        6 => 'testvalue2'
    ),
    3 => 'testvalue3',
    4 => 'testvalue4',
);

$iterator = new RecursiveIteratorIterator(
    new RecursiveArrayIterator($array),
    RecursiveIteratorIterator::SELF_FIRST
);

foreach ($iterator as $key => $value) {
    echo "$key => $value", PHP_EOL;
}

1 => Array
5 => Array
3 => testvalue1
2 => Array
6 => testvalue2
3 => testvalue3
4 => testvalue4