php 数组
Array
(
[0] => Array
(
[category_id] => 15
[category_name] => 14inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)

[1] => Array
(
[category_id] => 16
[category_name] => 15inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)

[2] => Array
(
[category_id] => 17
[category_name] => 16inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)

[3] => Array
(
[category_id] => 18
[category_name] => 17inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)

[4] => Array
(
[category_id] => 36
[category_name] => SUV
[parent_category_id] => 2
[parent_name] => 入タイヤ 品
)

[5] => Array
(
[category_id] => 37
[category_name] => VAN
[parent_category_id] => 2
[parent_name] => 入タイヤ 品
)

[6] => Array
(
[category_id] => 42
[category_name] => 16inch
[parent_category_id] => 87
[parent_name] => 国タイヤ 品
)

[7] => Array
(
[category_id] => 43
[category_name] => 17inch
[parent_category_id] => 87
[parent_name] => 国タイヤ 品
)

[8] => Array
(
[category_id] => 82
[category_name] => 24inch
[parent_category_id] => 2
[parent_name] => 入タイヤ 品
)

)

按照 parent_category_id 他的值进行分组。
比如：
Array
(
[0] => Array
( [0] => Array
(
[category_id] => 15
[category_name] => 14inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)

[1] => Array
(
[category_id] => 16
[category_name] => 15inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)

[2] => Array
(
[category_id] => 17
[category_name] => 16inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)

[3] => Array
(
[category_id] => 18
[category_name] => 17inch
[parent_category_id] => 1
[parent_name] => ホイル?タイヤセット
)
)

[1] => Array
(
[0] => Array
(
[category_id] => 36
[category_name] => SUV
[parent_category_id] => 2
[parent_name] => 入タイヤ 品
)

[1] => Array
(
[category_id] => 37
[category_name] => VAN
[parent_category_id] => 2
[parent_name] => 入タイヤ 品
)
[2] => Array
(
[category_id] => 82
[category_name] => 24inch

[parent_category_id] => 2
[parent_name] => 入タイヤ 品
)
)

[2] => Array
( [0] => Array
(
[category_id] => 42
[category_name] => 16inch
[parent_category_id] => 87
[parent_name] => 国タイヤ 品
)
[1] => Array
(
[category_id] => 43
[category_name] => 17inch
[parent_category_id] => 87
[parent_name] => 国タイヤ 品
)


)
)
代码简单点 效率高点 就行。

[解决办法]
foreach($arr AS $v)
{
$res[$v["parent_category_id"]][] = $v;
}

结果在$res数组中