Transform flat array into tree

flat2tree.php

<?php

// http://php.net/manual/en/language.references.php
$flat = array(
    "7" => array(
        "id" => 7,
        "parent" => 6
    ),
    "6" => array(
        "id" => 6,
        "parent" => 5
    ),
    "5" => array(
        "id" => 5,
        "parent" => 4
    ),
    "4" => array(
        "id" => 4,
        "parent" => 0
    ),
    "3" => array(
        "id" => 7,
        "parent" => 2
    ),
    "2" => array(
        "id" => 7,
        "parent" => 1
    ),
    "1" => array(
        "id" => 7,
        "parent" => 0
    )
);

# transform $flat into a tree:
foreach($flat as $id => &$value)
{
    # check if there is a patrent
    if ($parentId = $value['parent'])
    {
        $flat[$parentId][0][$id] =& $value; # add child to parent
        unset($flat[$id]); # remove from topmost level
    }
}
unset($value); # remove iterator reference
print_r($flat); # your tree

StackOverflow question:
http://stackoverflow.com/questions/7730889/hierarchy-commenting-system-php