PHP - How to build tree structure list?

You do not need to create 2 tables in the database for this you can maintain it like below from one table only

+-------+---------------+---------------------------+
|   id  |   parent_id   |           title           |
+-------+---------------+---------------------------+
|   1   |       0       |   Parent Page             |
|   2   |       1       |   Sub Page                |
|   3   |       2       |   Sub Sub Page            |
|   4   |       0       |   Another Parent Page     |
+-------+---------------+---------------------------+

The array generated will be like

Array
(
    [0] => Array
        (
            [id] => 1
            [parent_id] => 0
            [title] => Parent Page
            [children] => Array
                        (
                            [0] => Array
                                (
                                    [id] => 2
                                    [parent_id] => 1
                                    [title] => Sub Page
                                    [children] => Array
                                                (
                                                    [0] => Array
                                                        (
                                                            [id] => 3
                                                            [parent_id] => 1
                                                            [title] => Sub Sub Page
                                                        )
                                                )
                                )
                        )
        )
    [1] => Array
        (
            [id] => 4
            [parent_id] => 0
            [title] => Another Parent Page
        )
)

You need to use the below recursive function to achieve it

function buildTree(array $elements, $parentId = 0) {
    $branch = array();

    foreach ($elements as $element) {
        if ($element['parent_id'] == $parentId) {
            $children = buildTree($elements, $element['id']);
            if ($children) {
                $element['children'] = $children;
            }
            $branch[] = $element;
        }
    }

    return $branch;
}

$tree = buildTree($rows);

The algorithm is pretty simple:

1. Take the array of all elements and the id of the current parent (initially 0/nothing/null/whatever).
2. Loop through all elements.
3. If the parent_id of an element matches the current parent id you got in 1., the element is a child of the parent. Put it in your list of current children (here: $branch).
4. Call the function recursively with the id of the element you have just identified in 3., i.e. find all children of that element, and add them as children element.
5. Return your list of found children.

$ritit = new RecursiveIteratorIterator(new RecursiveDirectoryIterator(__DIR__), RecursiveIteratorIterator::CHILD_FIRST);
$r = array();
foreach ($ritit as $splFileInfo) {
   $path = $splFileInfo->isDir()
         ? array($splFileInfo->getFilename() => array())
         : array($splFileInfo->getFilename());

   for ($depth = $ritit->getDepth() - 1; $depth >= 0; $depth--) {
       $path = array($ritit->getSubIterator($depth)->current()->getFilename() => $path);
   }
   $r = array_merge_recursive($r, $path);
}

print_r($r);

The code is rather poorly formatted. In the first part of your question, it's not at all clear why

((atom tree) (list tree))

would appear in any Common Lisp code, since it looks like an attempt to call (atom tree), get a function back, and call that function with (list tree). In context, with proper formatting, it's clearer:

(defun flatten (tree)
  (cond 
   ((null tree) nil)
   ((atom tree)(list tree))
   (t (loop for a in tree
            appending (flatten a)))))

It's one clause in a cond. It says that if tree is an atom (which could be a symbol, a number, a vector, etc., anything else that isn't a cons), then return (list tree). As larsmans explained, flatten should always return a list, and this ensures that (flatten 3), for instance, will return (3). Since the (loop for a in tree ...) will work for any tree that is a list, there's really no need to have a case for (null tree), since nil/() is a list. This definition could be simplified to:

(defun flatten (tree)
  (if (atom tree)
      (list tree)
      (loop for a in tree
            appending (flatten a)))))

There are some similar questions out there on , and some have almost identical code to what you posted (modulo the formatting). Did the friend was basing a suggestion on one of those. At any rate, for the sake of completeness, you might have a look at:

• flatten list in lisp
• Flatten Nests Function in Lisp - need help understanding (almost identical code)
• Common Lisp - flatting a list that may contain symbols (similar code)