How to form a json request correctly?

Question

PHP

Problem with generating JSON request
It should turn out:

{
  "arr": [
    "3333"
  ]
}

I write like this:

$fields= [
  'arr' => [ '3333']
];
echo json_encode($fields)

It looks like this (and an error):

{
  "arr": [
    0 = "3333"
  ]
}

Answer 1

what?