Placeholders in url for custom tab don't work.

I try to implement already build app in PHP to Teams custom tab. Page works fine but I need some data from teams. All data I need are provided in placeholders as noted here: 

https://docs.microsoft.com/en-us/microsoftteams/platform/tabs/how-to/access-teams-context

The problem is that if I insert any placeholder like ?tid={tid} I get a blank page. I can't find a way to debug this to find what's wrong. 

Basically I have a simple PHP applications where I need to read $_GET data but for some reason when I add any placeholder I get a blank tab. Anyone had similar issues ?