How to create multi-dimensional array from a list?

Question

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.

what s your key to array? how do you want your array to look? do you want assoc. array? or you want to store each row as an object? — DarthVader
– DarthVader, Commented Oct 25, 2011 at 3:24

deceze · Accepted Answer · 2011-10-25 03:33:33Z

5

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.

answered Oct 25, 2011 at 3:33

deceze♦

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3 Comments

lsl Over a year ago

$parent = null should be $parent = "" mysql returns empty string for null values. Other than that +1.

deceze Over a year ago

@Louis You're right. It'll still work correctly though, since we're only comparing loosely. :)

Yoshi Over a year ago

@deceze You can do this with one loop only, no need for n*n loops. ;)

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