How can I retrieve the full directory tree using SPL?

By default, the RecursiveIteratorIterator will use LEAVES_ONLY for the second argument to __construct. This means it will return files only. If you want to include files and directories (at least that's what I'd consider a full directory tree), you'd have to do:

$iterator = new RecursiveIteratorIterator(
    new RecursiveDirectoryIterator($path),
    RecursiveIteratorIterator::SELF_FIRST
);

and then you can foreach over it. If you want to return the directory tree instead of outputting it, you can store it in an array, e.g.

foreach ($iterator as $fileObject) {
    $files[] = $fileObject;
    // or if you only want the filenames
    $files[] = $fileObject->getPathname();
}

You can also create the array of $fileObjects without the foreach by doing:

$files[] = iterator_to_array($iterator);

If you only want directories returned, foreach over the $iterator like this:

foreach ($iterator as $fileObject) {
    if ($fileObject->isDir()) {
        $files[] = $fileObject;
    }
}

You can just, or do everythng that you want

foreach(new RecursiveIteratorIterator(new RecursiveDirectoryIterator($path)) as $file)
{
    /* @var $file SplFileInfo */
    //...
}