json adding into a specific place

Question

Below is the JSON code How to inject with PHP a new part(in this case a new slide inside the "slides":{. This needs to be placed at the bottom (see --- NEW PART HERE ---)

The JSON comes from the database

Any help would be appreciated

{
   "settings":{

   },
   "slides":[
      {
         "title":"Demo",
      },
      {
        --- NEW PART HERE ("title":"Demo2") ---
      }
   ]
}