Proximity beacon API, Invalid JSON payload received

Question

Can someone help me with this. I'm new to Eddystone Beacon and this is my first time to use a Google Cloud API (Proximity beacon api). I'm trying to add a Nearby Notification to one of my registered beacons (that can be done in google beacon dashboard) via Proximity Beacon API and wondering why am I getting this error:

{ "error": { "code": 400, "message": "Invalid JSON payload received. Unknown name \"{\"attachmentName\":\"beacon_eddystone\",\"data\":\"eyJ0aXRsZSI6IlNJTSBUZXN0IiwidXJsIjoiaHR0cDpcL1wvd3d3LnN1cmlnYW9pbnRlcm5ldG1hcmtldGluZy5jb20ifQ\": Cannot bind query parameter. Field '{\"attachmentName\":\"beacon_eddystone\",\"data\":\"eyJ0aXRsZSI6IlNJTSBUZXN0IiwidXJsIjoiaHR0cDpcL1wvd3d3LnN1cmlnYW9pbnRlcm5ldG1hcmtldGluZy5jb20ifQ' could not be found in request message.", "status": "INVALID_ARGUMENT", "details": [ { "@type": "type.googleapis.com/google.rpc.BadRequest","fieldViolations": [ { "description": "Invalid JSON payload received. Unknown name \"{\"attachmentName\":\"beacon_eddystone\",\"data\":\"eyJ0aXRsZSI6IlNJTSBUZXN0IiwidXJsIjoiaHR0cDpcL1wvd3d3LnN1cmlnYW9pbnRlcm5ldG1hcmtldGluZy5jb20ifQ\": Cannot bind query parameter. Field '{\"attachmentName\":\"beacon_eddystone\",\"data\":\"eyJ0aXRsZSI6IlNJTSBUZXN0IiwidXJsIjoiaHR0cDpcL1wvd3d3LnN1cmlnYW9pbnRlcm5ldG1hcmtldGluZy5jb20ifQ' could not be found in request message." } ] } ] } } 

I made an HttpRequest using cURL in php and here's the code:

$array = array();
$array['title'] = "SIM Test";
$array['url'] = "http://www.surigaointernetmarketing.com";
$data = base64_encode(json_encode($array));

//echo $data;

$curl = curl_init(); 

curl_setopt_array($curl, array( 
    CURLOPT_URL => 'https://proximitybeacon.googleapis.com/v1beta1/beacons/<my-beacon-name>/attachments?projectId=<my-project-id>', 
    CURLOPT_RETURNTRANSFER => true, 
    CURLOPT_ENCODING => "", 
    CURLOPT_MAXREDIRS => 10, 
    CURLOPT_TIMEOUT => 30, 
    CURLOPT_HTTP_VERSION => 
    CURL_HTTP_VERSION_1_1, 
    CURLOPT_CUSTOMREQUEST => 'POST', 
    CURLOPT_POSTFIELDS => '{"attachmentName":"beacon_eddystone","data":"'.$data.'","namespacedType":"com.google.nearby/en"}', 
    CURLOPT_HTTPHEADER => array( "authorization: Bearer <my-bearer-token>" ),)); 

$response = curl_exec($curl); 
$err = curl_error($curl); 
curl_close($curl); 

if ($err)
    echo "cURL Error #:" . $err; 
else
    echo $response;

I got the code from here. And followed the instructions in the Get Started with the Proximity Beacon REST API. There must be a mistake in the http request body (CURLOPT_POSTFIELDS). And I couldn't figure it out why, please help.