PayPal Adaptive Payments Certificate Error - php

Question

I am trying to implement PayPal Adaptive Payments, but as soon as I try to post with cURL, I get a NULL response and curl_error() gives the following error:
0NSS: client certificate not found (nickname not specified)
Here is the full PHP code, I am using sandbox. Some sensitive info is starred (*).
<?php
class PayPal {
public $API_USER = "*********.gmail.com";
public $API_PASSWORD = "********************";
public $API_SIGNATURE = "******************************";
public $API_APP_ID = "APP-80W284485P519543T";
public $apiUrl = "https://svcs.sandbox.paypal.com/AdaptivePayments/";
public $paypalUrl = "https://sandbox.paypal.com/webscr?cmd=ap-payment&paykey=";
public $headers = array();
function __construct() {
$this->headers = array(
"X-PAYPAL-SECURITY-USERID : " . $this->API_USER,
"X-PAYPAL-SECURITY-PASSWORD : " . $this->API_PASSWORD,
"X-PAYPAL-SECURITY-SIGNATURE : " . $this->API_SIGNATURE,
"X-PAYPAL-REQUEST-DATA-FORMAT : JSON",
"X-PAYPAL-RESPONSE-DATA-FORMAT : JSON",
"X-PAYPAL-APPLICATION-ID : " . $this->API_APP_ID
);
}
function paypalSend($data, $call) {
$ch = curl_init();
$curl_params = array(
CURLOPT_URL => $this->apiUrl . $call,
CURLOPT_RETURNTRANSFER => TRUE,
CURLOPT_SSL_VERIFYPEER => FALSE,
CURLOPT_SSL_VERIFYHOST => FALSE,
CURLOPT_HTTPHEADER => $this->headers,
CURLOPT_POSTFIELDS => json_encode($data)
);
curl_setopt_array($ch, $curl_params);
$result = curl_exec($ch);
echo(curl_errno($ch));
echo(curl_error($ch));
curl_close($ch);
return json_decode($result, TRUE);
}
function getPayKey() {
$createPacket = array(
"actionType" => "PAY",
"currencyCode" => "USD",
"receiverList" => array(
"receiver" => array(
array(
"amount" => "5.00",
"email" => "*********#gmail.com"
)
)
),
"returnUrl" => "http://example.com/success",
"cancelUrl" => "http://example.com/failure",
"requestEnvelope" => array(
"errorLanguage" => "en_US",
"detailLevel" => "ReturnAll"
)
);
// paypal Send
$response = $this->paypalSend($createPacket, "Pay");
var_dump($response);
}
}
$pp = new Paypal();
$pp->getPayKey();
?>