php setting array index to value of an ID

I am having an issue where I am trying to save an ID as an array index, but when I convert the array to json, the ID is appearing as an object and not as an array element.

This is a brief example:

    $ID = 1001 ;
    $item = [] ;
    $item["regional"] = [] ;
    $item["regional"][$ID] = $someInfo ; //object
    json_encode($item) ;

I am returning the json to my app, and when I look at the json formatting the above lists 1001 as an object:

     [0: 
         { regional:
              { 1001 :
                   someInfoA : valueA
                   someInfoB : valueB
              }
         }
    ]

If I change the value of $ID = 0, then it stores the ID as a proper array index - which is what I am needing:

    $ID = 0 ;
    $item = [] ;
    $item["regional"] = [] ;
    $item["regional"][$ID] = $someInfo ; //object
    json_encode($item) ;

The above now shows ID = 0 as an array index [0]:

     [0: 
         { regional:
              [ 0 :
                   {
                   someInfoA : valueA
                   someInfoB : valueB
                   }
              ]
         }
    ] 

How can I get the first example to set the array index as 1001 and be treated as an array element versus as an object. I am needing to do this so I can quickly see if that array index element/ID exists to either add to it, or create it then add to it.

If this can't be done, then how can I search a multi-dimensional array/object to return the array index that contains the object value 1001 ?

     [0: 
         { regional:
              [ 0 :
                   {
                   ID: 1000
                   someInfoA : valueA
                   someInfoB : valueB
                   }
              ]
              [ 1 :
                   {
                   ID: 1001
                   someInfoA : valueA
                   someInfoB : valueB
                   }
              ]
         }
    ] 

$index = array_key_exists($item["regional"],1001) ;  // or something like this
var_dump($index) ; // index = 1 ;