PHP CURL Multidimensional Post

Can someone possibly tell me what's wrong with this? I've never tried to post multidim array over curl/php, any help would be mega appreciated.

Getting bad response from server, I think they're getting some of the data but their response isn't helpful. Trying to build on their code, so they won't help with this kind of thing.

$post_data = array (
    'tradeOrder' => array (
        'FirstOrder' => array (
            'Legs' => array (
                'Id' => '0',
                'SecurityId' => '643',
                'SecurityName' => 'AAPL',
                'SecurityExchange' => 'NASDAQ national market',
                'Side' => 'Buy',
                'Quantity' => '100'
            ),
            'SymbolLastPrice' => '93.72',
            'Price' => '93.75',
            'StopPrice' => '0.01',
            'Type' => 'Limit',
            'TimeInForce' => 'Day',
            'AllOrNone' => 'false',
            'Exchange' => 'AUTO',
            'TrailingAmount' => '0.01',
            'TrailingAmountType' => 'Absolute',
            'LimitOffset' => '0',
            'LimitOffsetType' => 'Absolute',
            'IsTrailingVisible' => 'false',
            'IsLimitOffsetVisible' => 'false',
            'IsTrailingAmountAbsolute' => 'true',
            'IsLimitOffsetAbsolute' => 'true',
            'ExecutionTarget' => 'New',
        ),
    ),
    'AdvancedOrderType' => 'Simple',
    'IsAdvancedOrder' => 'false'
);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://demo-trading.just2trade.com/TradeTicket/PlaceOrder");
curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
//curl_setopt($ch, CURLOPT_HEADER, true);
//curl_setopt($ch, CURLOPT_HTTPHEADER, ['Authorization: Bearer ' . $result_array['access_token']]);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($post_data));
curl_setopt($ch, CURLOPT_COOKIEJAR, $cookie_jar);
curl_setopt($ch, CURLOPT_COOKIEFILE, $cookie_jar);
curl_setopt($ch, CURLOPT_ENCODING,'gzip');
curl_setopt($ch, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4 );
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);

Forms I'm trying to post as follows

{
    "tradeOrder": {
        "FirstOrder": {
            "Legs": [{
                "Id": 0,
                "SecurityId": 643,
                "SecurityName": "AAPL",
                "SecurityExchange": "NASDAQ national market",
                "Side": "Buy",
                "Quantity": 100
            }],
            "SymbolLastPrice": 93.72,
            "Price": 93.72,
            "StopPrice": 0.01,
            "Type": "Limit",
            "TimeInForce": "Day",
            "AllOrNone": false,
            "Exchange": "AUTO",
            "TrailingAmount": 0.01,
            "TrailingAmountType": "Absolute",
            "LimitOffset": 0,
            "LimitOffsetType": "Absolute",
            "IsTrailingVisible": false,
            "IsLimitOffsetVisible": false,
            "IsTrailingAmountAbsolute": true,
            "IsLimitOffsetAbsolute": true,
            "ExecutionTarget": "New"
        },
        "AdvancedOrderType": "Simple",
        "IsAdvancedOrder": false
    }
}