http://zend-framework-community.634137.n4.nabble.com/create-tree-from-arrays-td2197351.html

mktree_array.example.php

<?php

$arr = array(
    array(
        'nid' => 1,
        'parentID' => 0
    ),
    array(
        'nid' => 2,
        'parentID' => 1
    ),
    array(
        'nid' => 3,
        'parentID' => 2
    )
);

print_r(mktree_array($arr));

mktree_array.php

<?php

/**
 * @param   array $arr Array to process
 * @param   integer $id (optional) Base parent ID
 * @return  array
 */
function mktree_array(&$arr, $id = 0)
{
    $result = array();
    foreach ($arr as $a) {
        if ($id == $a['parentID']) {
            $a['children']  = mktree_array($arr, $a['nid']);
            $result[] = $a;
        }
    }
    return $result;
}

output

Array
(
    [0] => Array
        (
            [nid] => 1
            [parentID] => 0
            [children] => Array
                (
                    [0] => Array
                        (
                            [nid] => 2
                            [parentID] => 1
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [nid] => 3
                                            [parentID] => 2
                                            [children] => Array
                                                (
                                                )

                                        )

                                )

                        )

                )

        )

)