Urban Airship push: Response: Got negative response from server: 0

I am trying to send push notification to my Android app from my server. But it is throwing error Payload: {"audience":"all","notification":{"android":{"alert":"PHP script test "}},"device_types":["android"]} Response: Got negative response from server: 0.

Below is the source code

<?php
  define('APPKEY','**************Mw'); // Your App Key
  define('PUSHSECRET','**********Low'); // Your Master Secret
  define('PUSHURL', 'https://go.urbanairship.com/api/push/');

  $contents = array();
  $contents['alert'] = "PHP script test";
  $notification = array();
  $notification['android'] = $contents;
  $platform = array();
  array_push($platform, "android");

  $push = array("audience"=>"all", "notification"=>$notification, "device_types"=>$platform);

  $json = json_encode($push);
  echo "Payload: " . $json . "\n"; //show the payload

  $session = curl_init(PUSHURL);
  curl_setopt($session, CURLOPT_USERPWD, APPKEY . ':' . PUSHSECRET);
  curl_setopt($session, CURLOPT_POST, True);
  curl_setopt($session, CURLOPT_POSTFIELDS, $json);
  curl_setopt($session, CURLOPT_HEADER, False);
  curl_setopt($session, CURLOPT_RETURNTRANSFER, True);
  curl_setopt($session, CURLOPT_HTTPHEADER, array('Content-Type:application/json', 'Accept: application/vnd.urbanairship+json; version=3;'));
 $content = curl_exec($session);
 echo "Response: " . $content . "\n";

 // Check if any error occured
 $response = curl_getinfo($session);
 if($response['http_code'] != 202) {
 echo "Got negative response from server: " . $response['http_code'] . "\n";
  } else {

 echo "Wow, it worked!\n";
 }

 curl_close($session);

?>

I am trying to run this php script from my browser. Push notification from urban airship server is working properly.

Thanks advance for any kind of help.