从我的数组树中添加前缀“>”

Array
(
[0] => Array
    (
        [id] => 39
        [parent_id] => 0
        [sku] => Parent
    )
[1] => Array
    (
        [id] => 40
        [parent_id] => 39
        [sku] => Child of parent id 39
    )
[2] => Array
    (
        [id] => 41
        [parent_id] => 40
        [sku] => Child of child id 40
    )
[3] => Array
    (
        [id] => 42
        [parent_id] => 40
        [sku] => Another child of child id 40
    )
[4] => Array
    (
        [id] => 43
        [parent_id] => 39
        [sku] => Another child of parent id 39
    )
etc..
)

我这里有一个关联数组，它基本上是父子关系。网上有很多关于如何构建一棵树的例子，这对我有很大帮助。但是现在我只想将我的数组格式化为这样的格式：

> Parent                               --> first prefix ">"
    >> Child of parent id 39           --> added two ">" prefixes
        >>> Child of child id 40       --> added three ">" prefixes
        >>> Another child of child id 40
    >> Another child of parent id 39
and so on..

更新：这就是我现在构建树的方式：

function pdf_content($data, $parentId = 0)
{
    $str = '';
    foreach ($data as $row) {
        if ($row['parent_id'] == $parentId) {
            $str .= '<li>';
            $str .= $row['sku'];
            $str .= pdf_content($data, $row['id']);
            $str .= '</li>';
        }
    }
    if ($str) {
        $str = "<ol class='dashed'>" . $str . '</ol>';
    }
    return $str;
}
// this is using the ol list style.

因此，首先，子项必须获取其父项并附加前缀">"。我已经有了一个构建树的代码，但是将其格式化为这样的东西让我被困在树上。