PHP Array to JSON Array

Question

I have the following PHP array:

array(3) {
  ["NDQ"]=>
  array(2) {
    [0]=>
    string(6) "Google"
    [1]=>
    string(5) "Yahoo"
  }
  ["NYSE"]=>
  array(2) {
    [0]=>
    string(5) "3M Co"
    [1]=>
    string(19) "Abbott Laboratories"
  }
  ["FX"]=>
  array(1) {
    [0]=>
    string(17) "Euro vs US Dollar"
  }
}

I want to save this as a JSON document so I do the follwoing:

$instrument_names = json_encode($instrument_names, JSON_FORCE_OBJECT);
file_put_contents("../data/instrument_names.json", $instrument_names);

But the JSON file I get is as follows:

{
    "NDQ":
    {
        "0":"Google",
        "1":"Yahoo"
    },

    "NYSE":
    {
        "0":"3M Co",
        "1":"Abbott Laboratories"
    },

    "FX":
    {
        "0":"Euro vs US Dollar"
    }
}

where each name has been given a key as the index of the array (which was sequentially numbered).

How do I turn this into an array instead?

"NDQ":
{
    ["Google","Yahoo"]
}

Answer 1

As someone kindly pointed out:

I was using JSON_FORCE_OBJECT option that forces sequential arrays to be JSON objects.

When I removed this, it worked fine.