Working PHP API Wrapper for Polonoiex

<?php
    class poloniex {
        protected $api_key;
        protected $api_secret;
        protected $base_url = "https://poloniex.com/tradingApi";
        protected $public_url = "https://poloniex.com/public";

        public function __construct($api_key, $api_secret) {
            $this->api_key = $api_key;
            $this->api_secret = $api_secret;
        }

        private function query($path, array $req = array()) {
            // API settings
            $key = $this->api_key;
            $secret = $this->api_secret;

            // generate a nonce to avoid problems with 32bit systems
            $mt = explode(' ', microtime());
            $req['nonce'] = $mt[1].substr($mt[0], 2, 6);

            // generate the POST data string
            $post_data = http_build_query($req, '', '&');
            $sign = hash_hmac('sha512', $post_data, $secret);

            // generate the extra headers
            $headers = array(
                'Key: '.$key,
                'Sign: '.$sign,
            );

            // curl handle (initialize if required)
            static $ch = null;
            if (is_null($ch)) {
                $ch = curl_init();
                curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
                curl_setopt($ch, CURLOPT_USERAGENT,
                    'Mozilla/4.0 (compatible; Poloniex PHP bot; '.php_uname('a').'; PHP/'.phpversion().')'
                );
            }
            curl_setopt($ch, CURLOPT_URL, $this->base_url.$path);
            curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);
            curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
            curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);

            // run the query
            $res = curl_exec($ch);

            if ($res === false) throw new Exception('Curl error: '.curl_error($ch));
            //echo $res;
            $dec = json_decode($res, true);
            if (!$dec){
                //throw new Exception('Invalid data: '.$res);
                return false;
            }else{
                return $dec;
            }
        }

        protected function retrieveJSON($URL) {
            $opts = array('http' =>
                array(
                    'method'  => 'GET',
                    'timeout' => 10
                )
            );
            $context = stream_context_create($opts);
            $feed = file_get_contents($URL, false, $context);
            $json = json_decode($feed, true);
            return $json;
        }

        public function get_balances() {
            return $this->query('',
                array(
                    'command' => 'returnBalances'
                )
            );
        }

        public function get_open_orders($pair) {
            // reverse pair
            $pair_arr = explode("_", $pair);
            $pair_reverse = strtoupper($pair_arr[1]."_".$pair_arr[0]);

            return $this->query('',
                array(
                    'command' => 'returnOpenOrders',
                    'currencyPair' => $pair_reverse
                )
            );
        }

        public function get_my_trade_history($pair) {
            return $this->query('',
                array(
                    'command' => 'returnTradeHistory',
                    'currencyPair' => $pair
                )
            );
        }

        public function buy($pair, $rate, $amount) {
            return $this->query('',
                array(
                    'command' => 'buy',
                    'currencyPair' => $pair,
                    'rate' => $rate,
                    'amount' => $amount
                )
            );
        }

        public function sell($pair, $rate, $amount) {
            return $this->query('',
                array(
                    'command' => 'sell',
                    'currencyPair' => $pair,
                    'rate' => $rate,
                    'amount' => $amount
                )
            );
        }

        public function cancel_order($pair, $order_number) {
            return $this->query('',
                array(
                    'command' => 'cancelOrder',
                    'currencyPair' => $pair,
                    'orderNumber' => $order_number
                )
            );
        }

        public function withdraw($currency, $amount, $address) {
            return $this->query('',
                array(
                    'command' => 'withdraw',
                    'currency' => $currency,
                    'amount' => $amount,
                    'address' => $address
                )
            );
        }

        public function get_total_btc_balance() {
            $results = $this->get_balances();
            $prices = $this->get_ticker();

            $tot_btc = 0;

            foreach($results as $coin => $amount){
                $pair_reverse = "BTC_".strtoupper($coin);

                if($amount > 0){
                    if($coin != "BTC"){
                        $tot_btc += $amount * $prices[$pair_reverse];
                    }else{
                        $tot_btc += $amount;
                    }
                }

                // get open orders
                if($coin != "BTC"){
                    $pair = strtoupper($coin."_btc");
                    $open_orders = $this->get_open_orders($pair);
                    foreach($open_orders as $order){
                        if($order['type'] == 'buy'){
                            $tot_btc += $order['total'];
                        }elseif($order['type'] == 'sell'){
                            $tot_btc += $order['amount'] * $prices[$pair_reverse];
                        }
                    }
                }
            }

            return $tot_btc;
        }

        public function get_ticker($pair = "all") {
            $prices = $this->retrieveJSON($this->public_url.'?command=returnTicker');
            if($pair == "all"){
                return $prices;
            }else{
                // reverse pair
                $pair_arr = explode("_", $pair);
                $pair_reverse = strtoupper($pair_arr[1]."_".$pair_arr[0]);
                if(isset($prices[$pair_reverse])){
                    $result = array();
                    $result['last'] = $prices[$pair_reverse];
                    return $result;
                }else{
                    return array();
                }
            }
        }

        public function get_trade_history($pair) {
            $trades = $this->retrieveJSON($this->public_url.'?command=returnTradeHistory&currencyPair='.$pair);
            return $trades;
        }

        public function get_order_book($pair) {
            $orders = $this->retrieveJSON($this->public_url.'?command=returnOrderBook&currencyPair='.$pair);
            return $orders;
        }

        public function get_volume() {
            $volume = $this->retrieveJSON($this->public_url.'?command=return24hVolume');
            return $volume;
        }
    }
?>