<?php
$source = array(
        'INBOX',
        'INBOX.lists',
        'INBOX.lists.asterisk',
        'INBOX.lists.asterisk.commits',
        'INBOX.lists.asterisk.bugs',
);


function buildMailFolderTree(&$folders, $folder, $path = '')
{
        $pos = strpos($folder, '.');
        if($pos !== false)
        {
                //subfolder
                $subFolders = substr($folder, $pos+1);
                $currentFolder = substr($folder, 0, $pos);
        }
        else
        {
                //toplevel folder
                $subFolders = false;
                $currentFolder = $folder;
        }

        $path .= $currentFolder;

        if(!isset($folders[$currentFolder]))
        {
                $folders[$currentFolder] = array(
                        'id' => $path,
                        'name' => $currentFolder,
                        'folders' => array(),
                );
        }
        else
        {
                // we have two folders with the same name ......
        }


        if(strlen($subFolders))
        {
                buildMailFolderTree($folders[$currentFolder]['folders'], $subFolders, $path . '.');
        }
}

$folders = array();
foreach($source as $folder)
{
        buildMailFolderTree($folders, $folder);
}

echo "folder structure ....\n";
var_dump($folders);
echo "\n\n";
?>


freebsd1% php ./foo.php
folder structure ....
array(1) {
  ["INBOX"]=>
  array(3) {
    ["id"]=>
    string(5) "INBOX"
    ["name"]=>
    string(5) "INBOX"
    ["folders"]=>
    array(1) {
      ["lists"]=>
      array(3) {
        ["id"]=>
        string(11) "INBOX.lists"
        ["name"]=>
        string(5) "lists"
        ["folders"]=>
        array(1) {
          ["asterisk"]=>
          array(3) {
            ["id"]=>
            string(20) "INBOX.lists.asterisk"
            ["name"]=>
            string(8) "asterisk"
            ["folders"]=>
            array(2) {
              ["commits"]=>
              array(3) {
                ["id"]=>
                string(28) "INBOX.lists.asterisk.commits"
                ["name"]=>
                string(7) "commits"
                ["folders"]=>
                array(0) {
                }
              }
              ["bugs"]=>
              array(3) {
                ["id"]=>
                string(25) "INBOX.lists.asterisk.bugs"
                ["name"]=>
                string(4) "bugs"
                ["folders"]=>
                array(0) {
                }
              }
            }
          }
        }
      }
    }
  }
}