Flat array to multilevel structure - php array

Question

Currently I have a table like so:

+-----------+---------------+-------------+-------------+
| category  | OrderID       | Name        | Name2       |
+-----------+---------------+-------------+-------------+
| ABC       | 12345         | Pen         | Black       |
+-----------+---------------+-------------+-------------+
| ABC       | 34545         | Pencil      | White       |   
+-----------+---------------+-------------+-------------+
| ABC       | 34545         | Pen         | Black       |
+-----------+---------------+-------------+-------------+
| DEF       | 12345         | Pencil      | Black       |
+-----------+---------------+-------------+-------------+
| DEF       | 12345         | Pen         | White       |
+-----------+---------------+-------------+-------------+

I like to have this in the tree format, I have read so many examples on ref. and recursive but didn't grasp the concepts. Can someone help?

Since the table will be huge - maybe around 20K records - not sure recursive would have any impact on the performance etc.

I have to have it done in this format due to the client html widget restrictions:

Array
(
[0] => Array
    (
        [category] => ABC
        [children] => Array
            (
            [0] => Array(
                [OrderID] => 12345
                [children] => Array
                    (
                        [Name] => Pen
                        [Name2] => Black

                    )
            )
            [1] => Array(
                [OrderID] => 34545
                [children] => Array
                (
                    [0] => Array(
                        [Name] => Pencil
                        [Name2] => White
                    )
                    [1] => Array(
                        [Name] => Pen
                        [Name2] => White
                    )
                )
            )

    )

[1] => Array
    (
        [category] => DEF
        [children] => Array
            (
                [OrderID] => 12345
                [children] => Array
                (
                    [0] => Array(
                        [Name] => Pencil
                        [Name2] => Black
                    )
                    [1] => Array(
                        [Name] => Pen
                        [Name2] => White
                    )
                )
            )
    )

Answer 1

Putting the value that need to be grouped into array content is not a good idea in many cases, because you can't use the value as lookup-key, thus making the grouping more difficult and inefficient.

$ori=json_decode(<<<JSON
[{"category":"ABC","orderID":"12345","Name":"Pen","Name2":"Black"},
{"category":"ABC","orderID":"34545","Name":"Pencil","Name2":"White"},
{"category":"ABC","orderID":"34545","Name":"Pen","Name2":"Black"},
{"category":"DEF","orderID":"12345","Name":"Pencil","Name2":"Black"},
{"category":"DEF","orderID":"12345","Name":"Pen","Name2":"White"}]
JSON
,true);
$result=array();
while($row=array_shift($ori))//Pretend that we're fetching record from DB
{
    $result[$row["category"]][$row["orderID"]][]=array("Name"=>$row["Name"],"Name2"=>$row["Name2"]);
}
print_r($result);

By using the grouped value (category and orderID) as array key, you make the finding and grouping more efficient. The above code prints out:

Array
(
    [ABC] => Array
        (
            [12345] => Array
                (
                    [0] => Array
                        (
                            [Name] => Pen
                            [Name2] => Black
                        )

                )

            [34545] => Array
                (
......

You still get the same count() results, and gets the benefit of finding certain category and order more easily: $result[category][order]["Name"]=....

But if you insist on getting your desired format, you can still add:

array_walk($result,function(&$v,$k){
    $x=array("category"=>$k,"children"=>array());
    array_walk($v,function($arr,$oid)use(&$x){
        $x["children"][]=array("orderID"=>$oid,"children"=>$arr);
    });
    $v=$x;
});
print_r(array_values($result));

This outputs:

Array
(
    [0] => Array
        (
            [category] => ABC
            [children] => Array
                (
                    [0] => Array
                        (
                            [orderID] => 12345
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [Name] => Pen
                                            [Name2] => Black
                                        )

                                )

                        )

                    [1] => Array
                        (
                            [orderID] => 34545
......

Demo. Anonymous function requires PHP>=5.3