Signing Twitter trends API v1.1 request in PHP fails


I’m struggling with signing an v1.1 request and put up my question already on StackOverflow here:
So I hope it’s okay if I don’t repost the whole text here?!
I really want to know why this is not working and where I went wrong. Because I think I followed Twitter’s documentation step by step.
Can anyone find the mistake?
Thanks so much!