$arr=[
      0 =>[
        "id" =>"2",
        "cate_name" =>"外用",
        "parent_id" => "0",
        "child" => [
          0 => [
            "id" => "5",
            "cate_name" => "外1",
            "parent_id" =>"2",
            "child" =>[
              0 => [
                "id" =>"7",
                "cate_name" => "外外1",
                "parent_id" => "5",
                "child" => NULL
               ]
            ]
          ],
          1=>[
            "id" => "6",
            "cate_name" => "外2",
            "parent_id" => "2",
            "child" => NULL,
          ]
        ]
      ],
      1 => [
        "id" => "3",
        "cate_name" =>"内用",
        "parent_id" => "0",
        "child" => NULL,
      ]
]

将这个三维数组分类树，组合成这种一维数组

$arr=[
    0=>["id" =>"2","cate_name" =>"外用","parent_id" => "0",],  //顶级分类
    1=>["id" => "5","cate_name" => "外1","parent_id" =>"2",],     //二级分类
    2=>["id" =>"7","cate_name" => "外外1","parent_id" => "5",],       //三级分类
    3=>["id" => "6","cate_name" => "外2","parent_id" => "2",],    //二级分类
    4=>["id" => "3","cate_name" =>"内用","parent_id" => "0",],//顶级分类
];

求大神们帮忙。