send post fields via curl with array structure [duplicate]

I would like so send post parameter via curl. I tried this:

curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(
    array(
       "content"            =>    "Hi",
       "message_type"       =>    "outgoing",
       "content_type"       =>    "input_select",
       "content_attributes" =>  '{ "items": [{ "title": "Option1", "value": "Option 1" }, { "title": "Option2", "value": "Option 2" }]}',
       "private"            =>    false
    )
));

But this way does not work. If works like this:

curl_setopt($ch, CURLOPT_POSTFIELDS, 
        '{
            "content": "Hi",
            "message_type" : "outgoing",
            "content_type": "input_select",
            "content_attributes": {
                "items": [
                    { "title": "Option1", "value": "Option1" },
                    { "title": "Option2", "value": "Option2" }
                ]
            },
            "private":false
        }'
    );

Would it be also possible to do it like the way with array (my first try). If yes, where is my mistake?