foreach ($coinNames_sites as $key => $coinNames_site){
    var_dump($key);
}

array_keys($coinNames_sites)

$json = <<<EOD
{
    "coinNames_btc38_cny": ["DOGE","XRP","BTS","NXT","BLK","XEM","DASH","PPC","XPM","RIC"],
    "coinNames_ybc_cny": ["aaa","bbb","ccc"]
}
EOD;

$coinNames_sites = json_decode($json);

foreach ($coinNames_sites as $coinNames_key => $coinNames_site){
    
    var_dump($coinNames_key);

}

？

$json = <<<EOD
{
    "coinNames_btc38_cny": ["DOGE","XRP","BTS","NXT","BLK","XEM","DASH","PPC","XPM","RIC"],
    "coinNames_ybc_cny": ["aaa","bbb","ccc"]
}
EOD;

$coinNames_sites = json_decode($json);
$keys = array_keys($coinNames_sites);
$coinName = $keys[1];
var_dump($coinName);