Send Test Invite to Candidate using HackerRank API - php

Question

I am using HackerRank as a test evaluation on my website. I am using it's API to send invite to my users for the test. I am calling it's API using CURL in PHP but i get error "invalid access token".
I use this code.
function httpPost($url,$params)
{
$postData = '';
//create name value pairs seperated by &
foreach($params as $k => $v)
{
$postData .= $k . '='.$v.'&';
}
$postData = rtrim($postData, '&');
$arr = array();
$ch = curl_init();
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
"Content-Type" => "application/x-www-form-urlencoded",
"Accept" => "application/json"
));
curl_setopt($ch,CURLOPT_URL,$url);
curl_setopt($ch,CURLOPT_RETURNTRANSFER,true);
curl_setopt($ch,CURLOPT_HEADER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch,CURLOPT_FOLLOWLOCATION,TRUE);
curl_setopt($ch,CURLOPT_MAXREDIRS,2);//only 2 redirects
curl_setopt($ch, CURLOPT_POST, count($postData));
curl_setopt($ch, CURLOPT_POSTFIELDS, $postData);
$output=curl_exec($ch);
if($output === false)
{
$arr[]= "Error Number:".curl_errno($ch)."<br>";
$arr[]= "Error String:".curl_error($ch);
}
curl_close($ch);
$arr[]= $output;
return $arr;
}
$data = array(
'test_id'=> test_id_integer,
'username'=> "EMAIL_ID",
'access_token'=> 'API_KEY'
);
var_dump(httpPost("www.hackerrank.com/x/api/v2/tests/test_id_integer/candidates",$data));