[php] How to downgrade php from 7.1.1 to 5.6 in xampp 7.1.1?

I know it might be late but I'm just adding to Lanti's answer since it's the most popular, I had the same problem as Wouter Vanherck in the comments and I can't comment yet.

What helped for me was instead of just replacing \xampp\apache\conf\extra\httpd-xampp.conf I replaced the whole apache folder. I basically did the same thing with it as with the php folder (steps 2 and 3).

Now the error is fixed and Apache starts just fine.