PHP：json_decode - 读取类别名称

I am trying to read JSON formatted prices from: https://poloniex.com/public?command=returnTicker

what I need are two things: symbol names (like BTC_BBR, BTC_BCN, etc) and "highestBid" price for them. To read the ladder I use something like this:

$polo_price = file_get_contents('https://poloniex.com/public?command=returnTicker');
$polo = json_decode($polo_price, true);
$symbol[1] =  $polo['BTC_BBR']['highestBid'];

But i need to use symbol name to read it. The symbols disapear and are being added from time to time, so I need to do this more automatically. How can I read symbol names into an array, so the result are:

symbol_name[0] = "BTC_BBR";
symbol_name[1] = "BTC_BCN";

and so on.

Try this:

<?php
$polo_price = file_get_contents('https://poloniex.com/public?command=returnTicker');
$polo = json_decode($polo_price, true);

foreach ($polo as $symbol=>$array){
    $symbols[]=$symbol;
    $highestBids[] = $array['highestBid'];
}

print_r($symbols);

print_r($highestBids);

<?php
function findHighestBid()
{
    $result = [];
    $url = "https://poloniex.com/public?command=returnTicker";
    $json_data = file_get_contents($url);
    $array_data = json_decode($json_data, true);
    foreach ($array_data as $currncy_option => $value) {
        $result[$currncy_option] = $value["highestBid"];
    }
    return $result;
}
print_r(findHighestBid());

It will automatically get the highest bid for all the currency option in the json data and also set currency as the key //output

Array
(
    [BTC_BBR] => 0.00026283
    [BTC_BCN] => 0.00000005
    [BTC_BELA] => 0.00002100
    [BTC_BITS] => 0.00000892
    [BTC_BLK] => 0.00003480
    [BTC_BTCD] => 0.00615298

Like this ....