PHP Browse a multidimensional array while preserving the keys

I've got a multidimensional array of which I can't know the depth. The array could for example look like this:

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

With this array I want to create a table of contents. That means the keys need to be preserved as I'm using them as "chapter numbers". For example, "testvalue1" is in chapter 1.5.3.
Now I want to walk through the array while preserving all keys - not using array_walk_recursive as the keys containing another array are dropped (correct?) and preferably not using nested foreach loops considering the speed.
Any suggestions how I should do this? Thanks in advance.

PS: For my script it doesn't matter if the keys are strings ("1" instead of 1) or integers, if having strings as key will make array_walk_recursive preserve them.


You can iterate over your array with a help of a stack to build your toc.

$stack = &$array;
$separator = '.';
$toc = array();

while ($stack) {
    list($key, $value) = each($stack);
    unset($stack[$key]);
    if (is_array($value)) {
        $build = array($key => ''); # numbering without a title.
        foreach ($value as $subKey => $node)
            $build[$key . $separator . $subKey] = $node;
        $stack = $build + $stack;
        continue;
    }
    $toc[$key] = $key. ' ' . $value;
}

print_r($toc);

Output:

Array
(
    [1] => 1
    [1.5] => 1.5
    [1.5.3] => 1.5.3 testvalue1
    [2] => 2
    [2.6] => 2.6 testvalue2
    [3] => 3 testvalue3
    [4] => 4 testvalue4
)

You can additionally handle the level as well if you need to, but that was not clear from your question.

array_walk_recursive does not work, because it won't give you the keys of the parent element(s). See this related question as well: Transparently flatten an array, it has a good answer and is helpful for more generic cases as well.