获取key的方法：

登录谷歌开发者后台 https://console.firebase.google.com/?hl=zh-cn

function firebaseNotice($title,$body){

  		$token_arr=['token1','token2']; //用户的firebasetoken列表
  		
        $notify_msg = [
            "notification" => [
                "title" => $title,
                "body" => $body
            ],
            "data" => [], //由开发者自己定义内容
            "registration_ids" => $token_arr,
            "direct_boot_ok"=>true
        ];

        $data = json_encode($notify_msg);
        $notify_server_key = 'AAAAk5ct8mI:A*******1LLtP6jWH'; //替换为自己的firebase开发者后台获取的key
        $url = "https://fcm.googleapis.com/fcm/send";
        $headers = [
            "Authorization:key=" . $notify_server_key,
            "Content-Type:application/json"
        ];
        try {
            $ch = curl_init();
            curl_setopt($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, $data);
            // 在尝试连接时等待的秒数
            curl_setopt($ch, CURLOPT_CONNECTTIMEOUT , 2);
            // 最大执行时间
            curl_setopt($ch, CURLOPT_TIMEOUT, 3);
            $result = curl_exec($ch);

            //var_dump($result);
            
            $respon = json_decode($result, true);
        } catch (\Exception $e) {
            $respon = [];
        }

        if (isset($respon['results'])) {
            if (isset($respon['results']['error'])) {
                return false;
            } else {
                return true;
            }
        }
        return true;
    }

调用方法：

$title='系统通知';
$body='通知内容通知内容';
firebaseNotice($title,$body);