Android Push Notification unauthorized error 401 - php

Question

I have created an Android app with Googles' OAuth 2.0 (from developer console) and has an API key for Android application (it use Google Maps, which works by the way).
However, my problem is that I'm using Push Notifications and it works on the android application though (I get the registration ID), but on the server it doesn't work at all.
I'm using PHP and the problem seems to be the API key which is the same as the one I've for the Android application.
So what key am I supposed to use? And how do I retrieve it?
Code:
<?php
function send_push_notification($registatoin_ids, $message) {
// Set POST variables
$url = 'https://android.googleapis.com/gcm/send';
$fields = array(
'registration_ids' => $registatoin_ids,
'data' => $message,
);
$headers = array(
'Authorization: key=An_Api_Key',
'Content-Type: application/json'
);
//print_r($headers);
// Open connection
$ch = curl_init();
// Set the url, number of POST vars, POST data
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
// Disabling SSL Certificate support temporarly
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
// Execute post
$result = curl_exec($ch);
if ($result === FALSE) {
die('Curl failed: ' . curl_error($ch));
}
// Close connection
curl_close($ch);
echo $result;
}?>

Answer 1

Please check allowed IP addresses in your Google API. You can restrict API access to certain IP addresses and by default one IP address is set when you create new API project.
Remove that IP address so that any IP address (Including your local machine's server) can connect and access the API.
I have implemented similar kind of code and got 401 error. Above fix worked for me. Hope that helps you as well.