php如何解析获取这种格式的json

{
    "usdt_cnyx": {
        "result": "true",
        "last": "6.912",
        "lowestAsk": "6.914",
        "highestBid": "6.912",
        "percentChange": "0.06",
        "baseVolume": "8082626.46086641",
        "quoteVolume": "1173640.89036410",
        "high24hr": "6.922",
        "low24hr": "6.874"
    },
    "btc_cnyx": {
        "result": "true",
        "last": "35718.67",
        "lowestAsk": "35732.01",
        "highestBid": "35669.06",
        "percentChange": "-0.81",
        "baseVolume": "330772.44399501",
        "quoteVolume": "9.20822579",
        "high24hr": "36975.3",
        "low24hr": "35353.09"
    },
    "eth_cnyx": {
        "result": "true",
        "last": "1177.24",
        "lowestAsk": "1174.33",
        "highestBid": "1172.98",
        "percentChange": "-5.02",
        "baseVolume": "566590.74282103",
        "quoteVolume": "478.24187550",
        "high24hr": "1249.77",
        "low24hr": "1152"
    }
}

我的代码：

$i=0;//循环变量
$num=count($obj);//得到Json数组长度

for ($i;$i<=$num;$i++)
{

    $symbol = $obj[$i];//这里要获取 usdt_cnyx这样的数据
    $last = $obj[$i]["last"];
    $high = $obj[$i]["high24hr"];
    $low = $obj[$i]["low24hr"];
    $bid = $obj[$i]["highestBid"];
    $ask = $obj[$i]["lowestAsk"];
    $percentchange = $obj[$i]["percentChange"];
    $vol = $obj[$i]["baseVolume"];
}

无法获取数据，请大神指教。

<?php
$txt = '
{
    "usdt_cnyx": {
        "result": "true",
        "last": "6.912",
        "lowestAsk": "6.914",
        "highestBid": "6.912",
        "percentChange": "0.06",
        "baseVolume": "8082626.46086641",
        "quoteVolume": "1173640.89036410",
        "high24hr": "6.922",
        "low24hr": "6.874"
    },
    "btc_cnyx": {
        "result": "true",
        "last": "35718.67",
        "lowestAsk": "35732.01",
        "highestBid": "35669.06",
        "percentChange": "-0.81",
        "baseVolume": "330772.44399501",
        "quoteVolume": "9.20822579",
        "high24hr": "36975.3",
        "low24hr": "35353.09"
    },
    "eth_cnyx": {
        "result": "true",
        "last": "1177.24",
        "lowestAsk": "1174.33",
        "highestBid": "1172.98",
        "percentChange": "-5.02",
        "baseVolume": "566590.74282103",
        "quoteVolume": "478.24187550",
        "high24hr": "1249.77",
        "low24hr": "1152"
    }
}';
$json = json_decode($txt, true);
foreach($json as $key=>$value){
    $last = $value["last"];
    $high = $value["high24hr"];
    $low = $value["low24hr"];
    $bid = $value["highestBid"];
    $ask = $value["lowestAsk"];
    $percentchange = $value["percentChange"];
    $vol = $value["baseVolume"];
}