How to create multi-dimensional array from a list?

I have a list of categories in MySQL with parent ID. How can I create a PHP array from the list.

ID  Category      Parent_ID
1   Car           NULL
2   Education     NULL
3   Mathematics   2
4   Physics       2
5   Astrophysics  4

I want to produce an array of this structure

array(
    "Car" => "1",
    "Education" => array("Mathematics" => "2", "Physics" => array("Astrophysics" => "4"))
);

As a matter of fact, key/value is not important as I will work with other columns too. I just want to know how to scan the list and produce multi-level list.

Solution 1:

Some very simple recursion to build a tree structure:

function buildTree(array $data, $parent = null) {
    $branch = array();

    foreach ($data as $row) {
        if ($row['parent_id'] == $parent) {
            $row['children'] = buildTree($data, $row['id']);
            $branch[] = $row;
        }
    }

    return $branch;
}

$tree = buildTree($rowsFromDatabase);

Having an explicit 'children' key is usually preferable to the structure you're proposing, but feel free to modify as needed.