Facebook Webhooks API

Not sure whether you guys can help me out here.

I have worked on CI for other project and it is working perfectly fine.

However, when I am trying to use CI for one of my Facebook Webhooks projects, it seems that CI is filtering the data posted from Facebook but if i create a standalone PHP file using the same code (except the GET/POST function which using CI $this->input), it is working fine.

When I enable CI debug mode, this is the debug log available:
DEBUG - 2017-05-12 02:18:51 --> Global POST, GET and COOKIE data sanitized

I have checked the HTTP log as well, HTTP 403 is returned for all Facebook post to CI.

I would really like to use CI for all my project. Hope someone out there will have some idea