convert json to array

laravel return array of objects how can i convert this array to array of arrays ? this is example

{  
   "data":[  
      {  
         "id":93,
         "created_at":"2017-12-12 07:47:34"
      },
      {  
         "id":78,
         "created_at":"2017-12-11 06:51:51"
      },
   ]
}

to 

{  
   "data":[  
      [  
         "id":93,
         "created_at":"2017-12-12 07:47:34"
      ],
      [  
         "id":78,
         "created_at":"2017-12-11 06:51:51"
      ],
   ]
}