I am getting some response as string(0) ""

public function requestToPay()
    {

        $url = "https://sandbox.momodeveloper.mtn.com/collection/v1_0/requesttopay";
        $curl = curl_init($url);

        curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "POST");
        curl_setopt($curl, CURLOPT_URL, $url);
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);

        # Request headers https://mysite.com
        $headers = array(
            'Authorization: '.$this->createAccessToken(),
            'X-Callback-Url: mysite.com',
            'X-Reference-Id: '.$this->getGUID(),
            'X-Target-Environment: sandbox',
            'Content-Type: application/json',
            'Cache-Control: no-cache',
            'Ocp-Apim-Subscription-Key: *******');
        curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);

        # Request body 0248888736
        $request_body = '{
            "amount": "5.0",
            "currency": "EUR",
            "externalId": "6353636",
            "payer": {
                "partyIdType": "MSISDN",
                "partyId": "46733123450"
            },
            "payerMessage": "Pay for product a",
            "payeeNote": "payer note"
        }';
        curl_setopt($curl, CURLOPT_POSTFIELDS, $request_body);

        $resp = curl_exec($curl);
        curl_close($curl);
        	$access_token = json_decode($resp,true);
        var_dump($resp);
        die();
}


 public function createAccessToken()
    {
        $auth=base64_encode($this->api_user.":".$this->api_key);
        // echo $auth; die();
    	$headers=array("Authorization: Basic ".$auth,
    	"Ocp-Apim-Subscription-Key:".$this->ocp_apim_subscription_key
    	);


        $url = 'https://sandbox.momodeveloper.mtn.com/collection/token/';
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
		curl_setopt($ch, CURLOPT_HTTPHEADER,$headers);
		curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
		curl_setopt($ch, CURLOPT_TIMEOUT, 50);
		curl_setopt($ch, CURLOPT_POST, true);
		curl_setopt($ch, CURLOPT_POSTFIELDS, array());
		curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
		$result = curl_exec($ch);
		$access_token = json_decode($result,true);
		curl_close($ch);

		return $access_token['access_token'];

    }

function getGUID(){
        if (function_exists('com_create_guid')){
            return com_create_guid();
        }else{
            mt_srand((double)microtime()*10000);//optional for php 4.2.0 and up.
            $charid = strtoupper(md5(uniqid(rand(), true)));
            $hyphen = chr(45);// "-"
            $uuid = substr($charid, 0, 8).$hyphen
                .substr($charid, 8, 4).$hyphen
                .substr($charid,12, 4).$hyphen
                .substr($charid,16, 4).$hyphen
                .substr($charid,20,12);

            return $uuid;
        }
    }

Can any one help me, where i am going wrong.

Thanks in Advance