Sorting comments into a multidimensional array using recursion PHP

I fetch my commments using MYSQL, with the format

comment_id, parent_id, group_id, message

Where the parent_id can be empty if the comment is a 1st level comment.

I store my nested comments, i.e replies in a child array, added within function that I have made.

Here is an output, including 1st level and 2nd level comments, basic comment/reply:

Array
(
    [0] => Array
        (
            [comment_id] => e465ce0a5301b8ed2eb66be06f768184f7727e3a
            [profile_id] => 8fa7a1679560876eaf2f8060abd916b692c719dc
            [name] => Chris Moore
            [parent_id] => 
            [comment] => You can do that, easy peasy!
            [type] => a
            [ambition_id] => 85c39f39553d4a004778b8936fb5084daa77c80d
            [registered] => 2013-11-19 14:34:41
            [child] => Array
                (
                    [0] => Array
                        (
                            [comment_id] => 68911c41a8cb13742dfd16f299aa3a2c9e87e16d
                            [profile_id] => 1dd36ac747735a3ee8a1d47750e1515ab7ac0d53
                            [name] => James Boyd
                            [parent_id] => e465ce0a5301b8ed2eb66be06f768184f7727e3a
                            [comment] => hello chris
                            [type] => a
                            [ambition_id] => 85c39f39553d4a004778b8936fb5084daa77c80d
                            [registered] => 2013-11-27 15:40:31
                        )

                    [1] => Array
                        (
                            [comment_id] => 7252cdab2c50dbb028e7b41f04bfb3fa7f6ff39d
                            [profile_id] => 8fa7a1679560876eaf2f8060abd916b692c719dc
                            [name] => Chris Moore
                            [parent_id] => e465ce0a5301b8ed2eb66be06f768184f7727e3a
                            [comment] => Test 14:17
                            [type] => a
                            [ambition_id] => 85c39f39553d4a004778b8936fb5084daa77c80d
                            [registered] => 2014-02-21 14:17:10
                        )
                )
        )
)

I have the following functions that I use to try and sort my comments, I manage to get 1st level and 2nd level comments working, but anything over that doesnt work.

Here are my current functions:

getCommentsForParent - a recursive call, to go from 2nd level to nth level replies and sort_comments - to setup the inital arrays and 1st level comments

function getCommentsForParent($p, $nested, $type){

    $index = 0;

    if(!empty($nested)){
        foreach($nested as $n){
            if($p['comment_id'] == $n['parent_id']){
                $p['child'][] = $n;
                $i = array_search($n, $nested);     
                unset($nested[$i]);
                $array = getCommentsForParent($n, $nested, 'inner');
                $n = $array[0];
                $nested = $array[1];
            }

            $index++;
        }
    }

    return array($p, $nested);
}

function sort_comments($ar){

    //split the comments into 1st level and nth level
    $parents = array();
    $nested = array();

    foreach ($ar as $item) {
        if(empty($item['parent_id'])){
            $parents[] = $item;
        }
        else{
            $nested[] = $item;
        }
    }

    if(is_array($parents)){
        $index = 0;
        foreach ($parents as $p) {
            if(!empty($nested)){
                $array = getCommentsForParent($p, $nested, 'parent');
                $p = $array[0];
                $nested = $array[1];
                $parents[$index] = $p;
            }
            $index++;
        }
    }

    return $parents;
}

Could you try and find a solution to my code, i'm sure that I am close.