In method refresh_token() it is being assumed that an array is being returned, which is not the case:

array_key_exists("access_token", $result)

Therefore it should rather be:

property_exists($result, "access_token")

Previously type-checking the $result could prevent passing the wrong data-type:

if (
    $result == null ||
    is_array($result) && !array_key_exists("access_token", $result) ||
    is_object($result) && !property_exists($result, "access_token")
)  {
    return null;
}

Hello,
I'm not PHP developer, I'm only able to modify and use foreign code :-) For this reason I will not send pull request :-)

There were problems with carriage return "%0D" char in $appid and $appsecret (probably caused by windows CRLF vs unix LF?)

url_https_post curl send body:grant_type=client_credentials&client_secret=blabla123blabla%0D&client_id=123456%0D

Solution in is trim() before usage.
Without this fix, the refresh token failed, result was null

Second problem:
it does not worked, it was not able to find and create instance of class TestPushMsgCommon()
include_once (dirname(__FILE__) . './push_common/test_sample_push_msg_common.php');
it works after change to:
include_once (dirname(__FILE__) . '/push_common/test_sample_push_msg_common.php');

environment:
Linux & PHP 5.2, macOS & PHP 7.1

test_sample_push_instantce_app_msg.php probably should be called test_sample_push_instant_app_msg.php - or quick_app?

Maybe I am a newb, but I can't locate that folder and the files in it and getting errors as a result:

/push_admin/push_msg/instanceapp/InstanceAppConfig.php
/push_admin/push_msg/instanceapp/InstanceAppPushbody.php
/push_admin/push_msg/instanceapp/InstanceAppRingtone.php

Where do you find those files or do you generate them somehow?

my push notification being treated as advertisement, and never alert and vibrate. my request body as below.

{ "validate_only": false, "message": { "android": { "notification": { "important": "HIGH", "title": "message title", "body": " message body", "click_action": { "type": 1, "intent": "#Intent;compo=com.rvr/.Activity;S.W=U;end" }, "style": 1, "buttons": [ { "name": "Learn more", "action_type": 1, "intent_type": 0, "intent": "intent://com.huawei.codelabpush/deeplink?#Intent;scheme=pushscheme;launchFlags=0x4000000;i.age=180;S.name=abc;end", "data": "{\"key1\":\"value1\",\"key2\":\"value2\",\"keyn\":\"valuen\"}" } ], "big_title": "HUAWEI Push Kit", "big_body": "HUAWEI Push Kit is a messaging service provided by Huawei for developers. It establishes a messaging channel from the cloud to devices." } }, "token": [ "my_token" ] } }