How to create a JSON object

Usually, you would do something like this:

$post_data = json_encode(array('item' => $post_data));

But, as it seems you want the output to be with "{}", you better make sure to force json_encode() to encode as object, by passing the JSON_FORCE_OBJECT constant.

$post_data = json_encode(array('item' => $post_data), JSON_FORCE_OBJECT);

"{}" brackets specify an object and "[]" are used for arrays according to JSON specification.

Although the other answers posted here work, I find the following approach more natural:

$obj = (object) [
    'aString' => 'some string',
    'anArray' => [ 1, 2, 3 ]
];

echo json_encode($obj);