json_encode : ignore numeric keys and formats

I need to convert a lot of hierchical arrays for an API format to send a requisition, but the document of the developer show different schemas of a natural PHP script, for example Items has a '[' and ']'

   "customerId":"xxxx",
   "scheduling":"2017-06-30T11:00:00-03:00",
   "items":[
      {
         "dimensions":{

And my code generate for example:

{ "customerId": "xxx", 
"scheduling": "2018-10-11T15:48:36-03:00", 
"items": { "0": 
{ "dimensions": 

Notice the {"0" after the Items array and too, the separator is { and not [

I merge all arrays using:

$post = json_encode(array_merge($a_header, $a_items, $a_newitem, $a_valueaddservice, $a_sender),JSON_PRETTY_PRINT);
$post = utf8_encode($post);

I'll need to write each chain of this format, or really there is a way to do it using arrays ?