Just get first json property?

I try to get the json property's from an url. But just the first property arrives.

        $json = file_get_contents('http://steamcommunity.com/market/priceoverview/?currency=3&appid=730&market_hash_name=' . $item_name);
        $obj = json_decode($json, true);
        $result = count($obj);
        echo $json;

Output is

"{"success":true}"

instead of

"{"success":true,"lowest_price":"0,96\u20ac","volume":"143","median_price":"0,89\u20ac"}"

What am i doing wrong?