private function sendPushNotification($fields) {
        // Set POST variables
        $url = 'https://fcm.googleapis.com/fcm/send';
        $headers = array(
            'Authorization: key=' . $this->api_key,
            'Content-Type: application/json'
        );
        // Open connection
        $ch = curl_init();
        curl_setopt( $ch,CURLOPT_URL, $url );
        curl_setopt( $ch,CURLOPT_POST, true );
        curl_setopt( $ch,CURLOPT_HTTPHEADER, $headers );
        curl_setopt( $ch,CURLOPT_RETURNTRANSFER, true );
        curl_setopt( $ch,CURLOPT_SSL_VERIFYPEER, false );
        curl_setopt( $ch,CURLOPT_POSTFIELDS, json_encode( $fields ) );
		curl_setopt($ch, CURLINFO_HEADER_OUT, true);
        // Execute post
		$info = curl_getinfo($ch);

        $result = curl_exec($ch);
        if ($result === FALSE) {
            die('Curl failed: ' . curl_error($ch));
        }
        // Close connection
        curl_close($ch);
        return $result;
    }